This adds more type safety to libgirepository, and allows
differentiating the `GIBaseInfo` derived types using the type system.
Two new derived types had to be added (previously they were just a
collection of helper methods which worked directly on a `GIBaseInfo` and
didn’t check types): `GICallbackInfo` and `GIUnresolvedInfo`.
Further cleanups and refactoring might be needed on this, but the core
of libgirepository now uses `GTypeInstance` and appears to still work
(it’s difficult to be entirely sure because there are no unit tests
yet).
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: #3155