The 20 most recently auto-saved games are kept in the file
DroidFish/pgn/.autosave.pgn.
Overwriting an existing game when saving a new game also auto-saves
the old game.
Add new setting (default true) to allow input of moves by dragging a
piece from its source square to the target square.
This setting partially overlaps with scroll gestures on the chess
board, so when this setting is enabled, scroll gestures will only
happen if the starting square is empty or contains a piece of the
color not to make the next move.
