Implement additional UDP connection on same port between client <> server.

Proof of concept containing:
- UDP Packets are linked to client sockets using random 64bit keys.
- UDP Packets have a sequence field, as the time can still yields collisions.
- Sequence fields are checked, and the short field is handled correctly with overflows.
- Only the SetPlayerData command is now transmitted over UDP.

Todo:
- Lag is not calculated on the UDP connection, but on the TCP connection.
- Calculate packet loss, and show indicator somewhere just like with FPS.
Merge pull request #19 from rayburgemeestre/network_multiplayer

Implement additional UDP connection on same port between client <> server