LibGUI: Simplify GCommonActions a bit

Use the same callback signature as GAction so we can just forward it
to GAction instead of chaining callbacks.

Applications/SystemMonitor/main.cpp

@@ -139,7 +139,7 @@ int main(int argc, char** argv)
 
     auto menubar = make<GMenuBar>();
     auto app_menu = make<GMenu>("System Monitor");
-    app_menu->add_action(GCommonActions::make_quit_action([] {
+    app_menu->add_action(GCommonActions::make_quit_action([](auto&) {
         GApplication::the().quit(0);
         return;
     }));