- use _NM_GET_PRIVATE() and _NM_GET_PRIVATE_PTR() everywhere.

- reorder statements, to have GObject related functions (init, dispose,
  constructed) at the bottom of each file and in a consistent order w.r.t.
  each other.

- unify whitespaces in signal and properties declarations.

- use NM_GOBJECT_PROPERTIES_DEFINE() and _notify()

- drop unused signal slots in class structures

- drop unused header files for device factories

- don't include "nm-default.h" in header files. Every source file must
  include as first header "nm-default.h", thus our headers get the
  default include already implicitly.

- we don't support compiling NetworkManager itself with a C++ compiler. Remove
  G_BEGIN_DECLS/G_END_DECLS from internal headers. We do however support
  users of libnm to use C++, thus they stay in public headers.

(cherry picked from commit f19aff89095ca192b8b2e37534b7a899aecd82f9)

Rather than randomly including one or more of <glib.h>,
<glib-object.h>, and <gio/gio.h> everywhere (and forgetting to include
"nm-glib-compat.h" most of the time), rename nm-glib-compat.h to
nm-glib.h, include <gio/gio.h> from there, and then change all .c
files in NM to include "nm-glib.h" rather than including the glib
headers directly.

(Public headers files still have to include the real glib headers,
since nm-glib.h isn't installed...)

Also, remove glib includes from header files that are already
including a base object header file (which must itself already include
the glib headers).

Most NMDevice types defined their own error domain but then never used
it. A few did use their errors, but some of those errors are redundant
with NMDeviceError, and others can be added to it.

Previously, src/nm-ip4-config.h, libnm/nm-ip4-config.h, and
libnm-glib/nm-ip4-config.h all used "NM_IP4_CONFIG_H" as an include
guard, which meant that nm-test-utils.h could not tell which of them
was being included (and so, eg, if you tried to include
nm-ip4-config.h in a libnm test, it would fail to compile because
nm-test-utils.h was referring to symbols in src/nm-ip4-config.h).

Fix this by changing the include guards in the non-API-stable parts of
the tree:

  - libnm-glib/nm-ip4-config.h remains   NM_IP4_CONFIG_H
  - libnm/nm-ip4-config.h now uses     __NM_IP4_CONFIG_H__
  - src/nm-ip4-config.h now uses       __NETWORKMANAGER_IP4_CONFIG_H__

And likewise for all other headers.

The two non-"nm"-prefixed headers, libnm/NetworkManager.h and
src/NetworkManagerUtils.h are now __NETWORKMANAGER_H__ and
__NETWORKMANAGER_UTILS_H__ respectively, which, while not entirely
consistent with the general scheme, do still mostly make sense in
isolation.

Rather than having NMManager know how to parse various settings to
create each kind of software device, add a _new_for_connection()
constructor to each of them and let them call NMPlatform to create the
device correctly themselves.

Software devices don't have a UDI until udev finds them, and since we need
to know about the software devices before udev finds them the UDI will be
missing.  Instead of requiring a UDI on NMDevice creation, update the
property from the NMPlatform link change signal when udev does find the
device.

Now that a UDI is no longer required for device creation, software devices
added by NM would be created in the platform_link_added_cb() signal
handler triggered by the various software device creation methods in
system_create_virtual_device() (eg nm_platform_bridge_add() etc).  Then
the NMDevice created in system_create_virtual_device() would be a duplicate
and cause problems when it was added.  Since system_create_virtual_device()
needs to do setup on some devices, suppress the device creation from the
platform link added handler in this function.

Much of this is a hack which should be cleaned up later.

Rather than passing UDI, ifname, and driver name to the device
constructors as separate arguments, just pass the NMPlatformLink
instead and let it parse them out.

Virtual types still take UDI and ifname separately, since we create
fake NMDevices for them for autoactivating connections. That's weird
in other ways too though, so perhaps this should be revisted.