This speeds up the initial object tree load significantly. Also, it
reduces the object management complexity by shifting the duties to
GDBusObjectManager.

The lifetime of all NMObjects is now managed by the NMClient via the
object manager. The NMClient creates the NMObjects for GDBus objects,
triggers the initialization and serves as an object registry (replaces
the nm-cache).

The ObjectManager uses the o.fd.DBus.ObjectManager API to learn of the
object creation, removal and property changes. It takes care of the
property changes so that we don't have to and lets us always see a
consistent object state.  Thus at the time we learn of a new object we
already know its properties.

The NMObject unfortunately can't be made synchronously initializable as
the NMRemoteConnection's settings are not managed with standard
o.fd.DBus Properties and ObjectManager APIs and thus are not known to
the ObjectManager.  Thus most of the asynchronous object property
changing code in nm-object.c is preserved. The objects notify the
properties that reference them of their initialization in from their
init_finish() methods, thus the asynchronously created objects are not
allowed to fail creation (or the dependees would wait forever). Not a
problem -- if a connection can't get its Settings, it's either invisible
or being removed (presumably we'd learn of the removal from the object
manager soon).

The NMObjects can't be created by the object manager itself, since we
can't determine the resulting object type in proxy_type() yet (we can't
tell from the name and can't access the interface list). Therefore the
GDBusObject is coupled with a NMObject later on.

Lastly, now that all the objects are managed by the object manager, the
NMRemoteSettings and NMManager go away when the daemon is stopped. The
complexity of dealing with calls to NMClient that would require any of
the resources that these objects manage (connection or device lists,
etc.) had to be moved to NMClient. The bright side is that his allows
for removal all of the daemon presence tracking from NMObject.

On D-Bus level, string (s) or object paths (o) cannot be NULL.
Thus, whenver server exposes such an object, it gets automatically
coerced to "" or "/", respectively.

On client side, libnm should coerce certain properties back, for which
"" is just not a sensible value.

For example, an empty NM_DEVICE_ETHERNET_HW_ADDRESS should be instead
exposed as NULL.

Technically, this is an API change. However, all users were well advised
to expect both NULL and "" as possible return values and handle them
accordingly.

sed 's#^/\*\{5\}\*\+/$#/*****************************************************************************/#' $(git grep -l '\*\{5\}' | grep '\.[hc]$') -i

- All internal source files (except "examples", which are not internal)
  should include "config.h" first. As also all internal source
  files should include "nm-default.h", let "config.h" be included
  by "nm-default.h" and include "nm-default.h" as first in every
  source file.
  We already wanted to include "nm-default.h" before other headers
  because it might contains some fixes (like "nm-glib.h" compatibility)
  that is required first.

- After including "nm-default.h", we optinally allow for including the
  corresponding header file for the source file at hand. The idea
  is to ensure that each header file is self contained.

- Don't include "config.h" or "nm-default.h" in any header file
  (except "nm-sd-adapt.h"). Public headers anyway must not include
  these headers, and internal headers are never included after
  "nm-default.h", as of the first previous point.

- Include all internal headers with quotes instead of angle brackets.
  In practice it doesn't matter, because in our public headers we must
  include other headers with angle brackets. As we use our public
  headers also to compile our interal source files, effectively the
  result must be the same. Still do it for consistency.

- Except for <config.h> itself. Include it with angle brackets as suggested by
  https://www.gnu.org/software/autoconf/manual/autoconf.html#Configuration-Headers

The localization headers are now included via "nm-default.h".

Also fixes several places, where we wrongly included <glib/gi18n-lib.h>
instead of <glib/gi18n.h>. For example under "clients/" directory.

Libraries need to include <gi18n-lib.h>, not <gi18n.h>, so that _()
will get defined to "dgettext (GETTEXT_DOMAIN, string)" rather than
"gettext (string)" (which will use the program's default domain, which
works fine for programs in the NetworkManager tree, but not for
external users). Likewise, we need to call bindtextdomain() so that
gettext can find the translations if the library is installed in a
different prefix from the program using it (and
bind_textdomain_codeset(), so it will know the translations are in
UTF-8 even if the locale isn't).

(The fact that no one noticed this was broken before is because the
libraries didn't really start returning useful translated strings much
until 0.9.10, and none of the out-of-tree clients have been updated to
actually show those strings to users yet.)

config.h should be included from every .c file, and it should be
included before any other include. Fix that.

(As a side effect of how I did this, this also changes us to
consistently use "config.h" rather than <config.h>. To the extent that
it matters [which is not much], quotes are more correct anyway, since
we're talking about a file in our own build tree, not a system
include.)

Add nm-core-types.h, typedefing all of the GObject types in
libnm-core; this is needed so that nm-setting.h can reference
NMConnection in addition to nm-connection.h referencing NMSetting.

Removing the cross-includes from the various headers causes lots of
fallout elsewhere. (In particular, nm-utils.h used to include
nm-connection.h, which included every setting header, so any file that
included nm-utils.h automatically got most of the rest of libnm-core
without needing to pay attention to specifics.) Fix this up by
including nm-core-internal.h from those files that are now missing
includes.

As with the settings, each device type was defining its own error
type, containing either redundant or non-useful error codes. Drop all
of the subtype-specific errors, and reduce things to just
NM_DEVICE_ERROR_FAILED, NM_DEVICE_ERROR_INCOMPATIBLE_CONNECTION, and
NM_DEVICE_ERROR_INVALID_CONNECTION.

The device-type-specific errors were only returned from their
nm_device_connection_compatible() implementations, so this is also a
good opportunity to simplify those, by moving duplicated functionality
into the base NMDevice implementation, and then allowing the
subclasses to assume that the connection has already been validated in
their own code. Most of the implementations now just check that the
connection has the correct type for the device (which can't be done at
the NMDevice level since some device types (eg, Ethernet) support
multiple connection types.)

Also, make sure that all of the error messages are localized.

Add _nm_object_class_add_interface(), for declaring that a class
implements a particular interface, and then have NMObject create the
corresponding proxies itself. (The subclass can get a copy with
_nm_object_get_proxy() if it needs it for something).

(In GDBus, creating a proxy is a heavier operation than in dbus-glib,
so we'll need to create the proxies asynchronously. Moving the
creation to NMObject makes that easier since we can do it as part
of the existing init/init_async.)