Otherwise the compositor gets all confused when it's trying to make
drag happen but we know it's not going to happen.
After all, we exchange data behind its back, we just need to keep it
informed.
We need to set the drag to NULL in gdk_wayland_drag_drop_done,
otherwise, all future drags will be considered local after the
first local one. Worse, they will also provide the wrong data.
We want to use a gdk_surface_new_popup for popups,
and align the constructor names with the surface
types, so rename
gdk_surface_new_popup -> gdk_surface_new_temp
gdk_surface_new_popup_full -> gdk_surface_new_popup
The temp surface type will disappear eventually.
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).
