A GObject interface, like a class, has two different C types
associated with it; the type of the "class" struct (eg, GObjectClass,
GFileIface), and the type of instances of that class/interface (eg,
GObject, GFile).

NetworkManager was doing this wrong though, and using the same C type
to point to both the interface's class struct and to instances of the
interface. This ends up not actually breaking anything, since for
interface types, the instance type is a non-dereferenceable dummy type
anyway. But it's wrong, since if, eg, NMDeviceFactory is a struct type
containing members "start", "device_added", etc, then you should not
be using an NMDeviceFactory* to point to an object that does not
contain those members.

Fix this by splitting NMDeviceFactory into NMDeviceFactoryInterface
and NMDeviceFactory; by splitting NMConnectionProvider into
NMConnectionProviderInterface and NMConnectionProvider; and by
splitting NMSettingsPlugin into NMSettingsPluginInterface and
NMSettingsPlugin; and then use the right types in the right places.

As a bonus, this also lets us now use G_DEFINE_INTERFACE.