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/LibImageDecoderClient/Client.h

@@ -41,7 +41,7 @@ private:
 
     virtual void die() override;
 
-    virtual void handle(const Messages::ImageDecoderClient::Dummy&) override;
+    virtual void dummy() override;
 };
 
 }