Two issues are addressed:
CORE-2189: missing undo feature
Works the same way as solitaire in windows xp:
- only 1 action can be undone;
- the player gets -2 points in standard score mode;
- the undo action resets when the player clicks on a row stack to turn the top card.
CORE-11148: invisible cards
This happens in 3-card mode, when only 1 card left in the deck. The fix for this is to modify the pile stack to contain all the face-up cards. It was actually already in the code somewhere else, so I turned it into a separate function.
