Userland: Change IPC funcs to use plain arguments instead of a struct

Instead of having a single overloaded handle method each method gets
its own unique method name now.

Userland/Libraries/LibGUI/Notification.cpp

@@ -34,7 +34,7 @@ private:
         , m_notification(notification)
     {
     }
-    virtual void handle(const Messages::NotificationClient::Dummy&) override { }
+    virtual void dummy() override { }
     Notification* m_notification;
 };