IPCCompiler: More work towards AudioServer bringup

- Add IEndpoint::handle(IMessage), a big switch table on message type.
  handle() will return a response message for synchronous messages,
  and return nullptr otherwise.
- Use i32 instead of int for everything
- Make IMessage::encode() const
- Make IEndpoint::decode_message() static, this allows template code to
  decode messages without an endpoint instance on hand.

Libraries/LibIPC/IMessage.h

@@ -9,7 +9,7 @@ public:
 
     virtual int id() const = 0;
     virtual String name() const = 0;
-    virtual ByteBuffer encode() = 0;
+    virtual ByteBuffer encode() const = 0;
 
 protected:
     IMessage();