[applets/fifteenPuzzle] Implement batch swaps on the same row/column

Usability / gameplay feature. The only one gameplay mechanic in this game is to move tiles. Player moves them by pressing on tiles directly beside an empty tile. But when player clicks on a tile that is on the same row or column as the empty tile, all tiles in between should move too.

See #8.

To be merged after the cleanup !169 (merged).