Add meta_ui_window_is_dummy()

For reasons related to interaction between the GTK+ CSS code and the
frame sync protocol, the dummy GtkWindow that MetaUI creates to track
theme properties has to be mapped and have MetaWindow associated with it.
Add a private function so that the test framework can filter this out.

https://bugzilla.gnome.org/show_bug.cgi?id=736505

src/ui/ui.c

@@ -653,3 +653,11 @@ meta_ui_window_is_widget (MetaUI *ui,
   else
     return FALSE;
 }
+
+gboolean
+meta_ui_window_is_dummy (MetaUI *ui,
+                         Window  xwindow)
+{
+  GdkWindow *frames_window = gtk_widget_get_window (GTK_WIDGET (ui->frames));
+  return xwindow == gdk_x11_window_get_xid (frames_window);
+}

src/ui/ui.h

@@ -116,5 +116,7 @@ gboolean meta_ui_have_a_theme      (void);
 
 gboolean meta_ui_window_is_widget (MetaUI *ui,
                                    Window  xwindow);
+gboolean meta_ui_window_is_dummy  (MetaUI *ui,
+                                   Window  xwindow);
 
 #endif