SpiceAgent: Let's start rewriting the messaging system :^)

The old message system was very dependent on syscalls, and the overall
structure made it hard to implement new features.

The new message system is pretty expandible, where each message has its
own dedicated class. As well as this, we now use Core::File and
AK::Stream for reading and writing messages.

Using AK::Stream also allows us to change the actual data source
(in this case, Core::File) without having to update a whole lot of code
in the future.

Userland/Services/SpiceAgent/CMakeLists.txt

@@ -5,6 +5,7 @@ serenity_component(
 
 set(SOURCES
     main.cpp
+    Message.cpp
     SpiceAgent.cpp
     ConnectionToClipboardServer.cpp
 )