Fixed adding moves to MovePagingList.
Added ChessGameWidget.

Move board moves_table out of ChessScene and into new class -
ChessGameWidget.
Fixed actions in MovePagingList.

* Added a layer of callbacks for next, previous, first, and last actions.

Added new actions for MovesWidget -- offerDraw and ResignGame.
Renamed files to end with _widget.py.

* Changed board.py and square.py to board_widget.py and square.py
  respectively.