Add "window-created" signal to MetaDisplay, "unmanaged" signal for MetaWindow

For some consumers it's significantly more convenient to be able
to directly connect to a signal on the Window to know when
Mutter is done with it, rather than having to connect to each
Workspace object (and handle workspace additions, etc.).

Similarly, add window-created which acts globally.

https://bugzilla.gnome.org/show_bug.cgi?id=598289

src/core/display-private.h

@@ -358,6 +358,9 @@ void        meta_display_register_x_window   (MetaDisplay *display,
 void        meta_display_unregister_x_window (MetaDisplay *display,
                                               Window       xwindow);
 
+void        meta_display_notify_window_created (MetaDisplay  *display,
+                                                MetaWindow   *window);
+
 GSList*     meta_display_list_windows        (MetaDisplay          *display,
                                               MetaListWindowsFlags  flags);