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).
As far as possible, use per-display debug flags.
This will minimize the debug spew that we get from
the inspector if it is running on a separate display.
