Special-case right-dragging of a numbered cell off the grid.

Special-case right-dragging of a numbered cell off the grid.

Chris Boyle reports that if you right-drag a numbered cell off the
grid, _all_ numbered cells (except the immutable initial clues) get
reset to blank, because this is treated as an attempt to erase a whole
chain of linked cells (of the form a,a+1,...,a_k) and the cells with
definite numbers are all treated as 'chain 0'.

In that situation, I now substitute the same behaviour you'd get by
left-dragging the numbered cell off the board, i.e. erase _just_ that
cell and not the whole of the rest of the puzzle.

(The previous unintended behaviour was a UI action you surely never
want - and Chris also reports that due to the Android front end's way
of representing right-drags, it's especially easy to hit by mistake.)