Added support for keeping track of high scores.  New menu item to
display the high scores - max of 10 scores per board size is kept.  The
scores are stored as json in a local file and reloaded when the the game
is restarted.  Player can now deselect a tile by touching the selected
tile again.  Numbers are displayed bigger and take the number of tiles
into account such that the numbers can be bigger for a 2x2 vs a 3x3,
etc.