Implement sync logic for multiplayer games

[Pyrdacor]

Client syncs his game by sending user actions (see #100). The server hosts the "real" game and syncs it when he or a client performs a user action. Otherwise he notice the clients that everything is still in sync.

The clients will create backup game states everytime a "sync" or "in-sync" message arrives. A sync is only a diff which can be small this way. A client may request a full game state sync (i.e. if the connection was lost for a few seconds).

Most of the logic works now. :) Still much to do. And much testing is required.