Make puzzle generator respect available clues

[turnerhayes]

Currently, if you change the layout of a puzzle (add or remove block cells), the selected cells may end up in an invalid state. For example, given the following layout:

(= represents a cell that is currently highlighted, X represents a block cell)

=	=	X
		X

The current direction is across, which is fine because there's a legitimate across clue. Now, if you add a block cell like so:

=	X	X
		X

then there is no longer a legitimate across clue, because the highlighted clue only has one cell across.

The app should automatically adjust the current direction to be down. If down is not valid, then some other behavior (remove cell selection?)