Added validity display.
Added checksumming.

The CIB checksum is calculated using PuzzleBuffer#checksum and
checked against the file value in PuzzleLoader#valid?
Added colour for background of highlighted cells.
Added button font.
Added highlighted to Cell. Added region to Clue.

Cell has a highlighted attribute.
Clues know where on screen they are currently listed.
The grid can now return a word's cell positions by its number and direction.
The crossword title is used.

Also, the first across word is highlighted.
Clue displaying has been re-factored.