* Change player map implentation in Game to EnumMap
* Add proper toString methods to Piece and MoveNode
* Compact code in MainWindow
* Synchronize methods in Game
* Enabled "next" and "previous" buttons
* Added corresponding methods to Board
* Added Move#invert()
* Rendering variation count in MoveNodeRenderer
The added functionality does not work correctly at the moment. This is
probably caused due to missing synchronization between the state of the
board, the game and the user interface.