GApplication: Add a way to replace a unique instance

While uniqueness is great, sometimes you want to restart
a newer version of the same app. These two flags make that
possible.

We also add a ::name-lost signal, that is emitted when it
happens. The default handler for this signal just calls
g_application_quit(), but applications may want to connect
and do cleanup or state-saving here.

gio/gapplication.h

@@ -112,9 +112,10 @@ struct _GApplicationClass
                                                      const gchar               *object_path);
   gint                      (* handle_local_options)(GApplication              *application,
                                                      GVariantDict              *options);
+  gboolean                  (* name_lost)           (GApplication              *application);
 
   /*< private >*/
-  gpointer padding[8];
+  gpointer padding[7];
 };
 
 GLIB_AVAILABLE_IN_ALL