Override GValue.set/get_boxed with static C marshaler

Override boxed type get/set methods on GValue to use the static C
GValue marshaler. This works around the inability of the introspection
version of these methods to know what the held GValue type is.
With this, all boxed types will now marshal properly with GValues as
their storage.

https://bugzilla.gnome.org/show_bug.cgi?id=688081