LibIPC+IPCCompiler: Add IPC::Decoder, let classes decode themselves

This shaves ~5 seconds off of a full build, not too bad. Also it just
seems nicer to push this logic out to classes. It could be better but
it's a start. :^)

Libraries/LibGfx/Color.h

@@ -278,3 +278,7 @@ const LogStream& operator<<(const LogStream&, Color);
 }
 
 using Gfx::Color;
+
+namespace IPC {
+bool decode(BufferStream&, Gfx::Color&);
+}