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/window-private.h

@@ -380,6 +380,7 @@ struct _MetaWindowClass
   void (*workspace_changed) (MetaWindow *window, int  old_workspace);
   void (*focus)             (MetaWindow *window);
   void (*raised)            (MetaWindow *window);
+  void (*unmanaged)         (MetaWindow *window);
 };
 
 /* These differ from window->has_foo_func in that they consider