generator: provide a GType static property for adapters

In the same way all GLib.Object-derived classes provided by the
generator have a GType static property (i.e. Gtk.Widget), we need
the same for *Adapter classes, to access their GType without having
an instance of it. (The first use case would be GStreamerSharp's
Gst.Bin.IterateAllByInterface ().)

For this, we cannot simply add the property to all adapter classes
and be done, because it would clash with a property which is already
there that has the same name and same value (this property is needed
for complying GInterfaceAdapter abstract class), so we rename this
property to GInterfaceGType because using this name for the static
one would not be consistent with the rest of the classes generated
which already provide the static one with the name "GType".