wayland: Invert drop_finished/button release call order

If we "release" the button first, the drag will be eventually cancelled,
we must first signal GDK_DROP_FINISHED, and then release the button so
the success status prevails.

gdk/wayland/gdkselection-wayland.c

@@ -779,9 +779,9 @@ data_source_send (void                  *data,
 
   if (context)
     {
-      gdk_wayland_device_unset_grab (gdk_drag_context_get_device (context));
       _gdk_wayland_drag_context_emit_event (context, GDK_DROP_FINISHED,
                                             GDK_CURRENT_TIME);
+      gdk_wayland_device_unset_grab (gdk_drag_context_get_device (context));
     }
 }