Add definition macro for internal types

Add definition macro for internal types

GTK defines various types that are meant to be derivable only within GTK
itself, and "final" from the perspective of consumers of the GTK API.
The existing macros defined by GObject, such as G_DECLARE_FINAL_TYPE and
G_DECLARE_DERIVABLE_TYPE, lack this functionality.

While we wait for GObject to get this kind of macro, we should define
our own.