Added meta_window_is_modal()

Queries whether the window is in a modal state, as per the _NET_WM_STATE
protocol.

src/include/window.h

@@ -116,4 +116,5 @@ void        meta_window_delete             (MetaWindow  *window,
                                             guint32      timestamp);
 int         meta_window_get_pid (MetaWindow *window);
 const char *meta_window_get_client_machine (MetaWindow *window);
+gboolean    meta_window_is_modal (MetaWindow *window);
 #endif