GLib master propagates argument types in g_clear_pointer(), which causes
the usual function pointer casts to GDestroyNotify to trip compiler
warnings. Additionally, this commit changes some cleanup functions where
appropriate (wl_data_source_destroy ->
gtk_primary_selection_source_destroy for struct
gtk_primary_selection_source).
This is to go along with the newly introduced GdkDrop.
This commit includes the necessary updates to the X11, Wayland
and Broadway backends. Other backends have to be updated separately.
