Server validation improvement

### (See commit messages)

* I've taken the liberty of implementing a more robust server
* I've also changed all clients, so that they can still play the game, however full support for new functionality is not yet added (like setting and change a player's name, error reporting, ... see the TicTacToeMessage enumeration class for a full list)
* To the client developers: please check my additions to the client side code!