Added FenWidget for graphically setting fen.

New widget sits next to palette squares to allow for settting
the turn, castle rights, en passant square, fifty move, and full
move fields of an enpassant square.

BoardWidget is now able to work in fen strings instead of just the
position part of the fen.
Fixed board updating logic with FenWidget.

A fen string is now passed to change positions.

Packed ChessGameWidget in a layout.

Mode button are now checkable and exclusive.