Add an optional pieces theme picker and another set of pieces as example

It might be useful to let players pick a chess piece theme in game. If this sounds desirable, I have a hackish proof of concept that could be developed into an optional dialog. [PyChess ](https://github.com/pychess/pychess/tree/master/pieces)has a diverse set of SVG pieces (and even TTF), we could convert some to PNG or investigate whether using SVG is viable.