Add an XDMCP implementation

Current status:

* Server:
 - Can refuse about 10k clients per second... that's all
 - It's implemented in Server.cpp (no established format yet, therefore no comments and messy), and Packet.cpp in onServerReceived methods.

* Client:
 - Didn't start working on it yet

* Common:
 - (De)marshalling all packet types should be done (encode(), decode() and constructors)