embedded-window: Set as app-paintable to workaround opaque region issues

In specific cases, GTK+ does not have enough information to set a correct
opaque region, in which the recommended fix is to set your window as
app-paintable. In the tray icon case, the socket window was considered
opaque but GTK+ as it had a solid window background, but it cannot have an
opaque region set, as the plug isn't composited against the socket, but
instead punches through the socket window.

https://bugzilla.gnome.org/show_bug.cgi?id=707614

src/shell-embedded-window.c

@@ -119,6 +119,7 @@ shell_embedded_window_constructor (GType                  gtype,
    * idle resize anyways.
    */
   g_object_set (object,
+                "app-paintable", TRUE,
                 "resize-mode", GTK_RESIZE_IMMEDIATE,
                 "type", GTK_WINDOW_POPUP,
                 NULL);