[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/window.c

@@ -1125,6 +1125,9 @@ meta_window_new_with_attrs (MetaDisplay       *display,
   if (window->wm_state_demands_attention)
     g_signal_emit_by_name (window->display, "window-demands-attention", window);
 
+  if (window->wm_hints_urgent)
+    g_signal_emit_by_name (window->display, "window-marked-urgent", window);
+
   return window;
 }