Port NMExportedObject to gdbus, and make
nm_exported_object_class_add_interface() deal with generating D-Bus
skeleton objects and attaching signal handlers and property bindings
as needed to properly handle methods, signals, and properties.
Our gdbus generated types use the same names as their corresponding
"real" types, but with "NM" changed to "NMDBus".
Unfortunately, that means that introspection/nmdbus-manager.c (the
generated type for src/nm-manager.c) uses the same type name as the
entirely unrelated src/nm-dbus-manager.c.
Fix this by removing the "d" from src/nm-dbus-manager.c. (We could
rename the generated type instead, but then it becomes inconsistent
with all the other generated types, and we're already using it as
"NMDBusManager" in libnm/nm-manager.c.)
Move D-Bus export/unexport handling into NMExportedObject and remove
type-specific export/get_path methods (export paths are now specified
at the class level, and NMExportedObject handles the counters for all
exported types automatically).
Since all exportable objects now use the same get_path() method, we
can also add some helper methods to simplify get_property()
implementations for object-path and object-path-array properties.
Add NMExportedObject, make it the base class of all D-Bus-exported
types, and move the nm-properties-changed-signal logic into it. (Also,
make NMSettings use the same properties-changed code as everything
else, which it was not previously doing, presumably for historical
reasons).
(This is mostly just shuffling code around at this point, but
NMExportedObject will be more important in the gdbus port, since
gdbus-codegen doesn't do a very good job of supporting objects that
export multiple interfaces [as each NMDevice subclass does, for
example], so we will need more glue/helper code in NMExportedObject
then.)
