https://bugzilla.gnome.org/show_bug.cgi?id=739127

Otherwise we might hit race conditions during build.

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.

callouts/tests/ needs -I$(top_builddir)/callouts to get
nmdbus-dispatcher.h

GLib-CRITICAL **: g_variant_type_checked_: assertion `g_variant_type_string_is_valid (type_string)' failed
GLib-CRITICAL **: g_variant_new_array: assertion `n_children > 0 || child_type != NULL' failed

https://bugzilla.gnome.org/show_bug.cgi?id=738654

gdbus-codegen will generate code that will need a too recent version of glib.

Port libnm-core/libnm to GDBus.

The NetworkManager daemon continues to use dbus-glib; the
previously-added connection hash/variant conversion methods are now
moved to NetworkManagerUtils (along with a few other utilities that
are now only needed by the daemon code).

In preparation for porting to GDBus, make nm_connection_to_dbus(),
etc, represent connections as GVariants of type 'a{sa{sv}}' rather
than as GHashTables-of-GHashTables-of-GValues.

This means we're constantly converting back and forth internally, but
this is just a stepping stone on the way to the full GDBus port, and
all of that code will go away again later.

Compile srcdir into test-dispatcher-envp rather than passing it on the
command-line, to make it easier to run by hand, and so we can use
automake's TESTS support.