Userland: Make IPC results with one return value available directly

This changes client methods so that they return the IPC response's
return value directly - instead of the response struct - for IPC
methods which only have a single return value.

Userland/Libraries/LibGUI/Menubar.cpp

@@ -30,7 +30,7 @@ Menu& Menubar::add_menu(String name)
 
 int Menubar::realize_menubar()
 {
-    return WindowServerConnection::the().create_menubar().menubar_id();
+    return WindowServerConnection::the().create_menubar();
 }
 
 void Menubar::unrealize_menubar()