[MetaDisplay] added "window-marked-urgent" signal

Having a MetaDisplay window-marked-urgent signal when a window sets its urgent
hint allows for centralized processing

https://bugzilla.gnome.org/show_bug.cgi?id=600068

src/core/display.c

@@ -131,6 +131,7 @@ enum
   FOCUS_WINDOW,
   WINDOW_CREATED,
   WINDOW_DEMANDS_ATTENTION,
+  WINDOW_MARKED_URGENT,
   LAST_SIGNAL
 };
 
@@ -246,6 +247,16 @@ meta_display_class_init (MetaDisplayClass *klass)
                   g_cclosure_marshal_VOID__OBJECT,
                   G_TYPE_NONE, 1, META_TYPE_WINDOW);
 
+  display_signals[WINDOW_MARKED_URGENT] =
+    g_signal_new ("window-marked-urgent",
+                  G_TYPE_FROM_CLASS (klass),
+                  G_SIGNAL_RUN_LAST,
+                  0,
+                  NULL, NULL,
+                  g_cclosure_marshal_VOID__OBJECT,
+                  G_TYPE_NONE, 1,
+                  META_TYPE_WINDOW);
+
   g_object_class_install_property (object_class,
                                    PROP_FOCUS_WINDOW,
                                    g_param_spec_object ("focus-window",