Sukoro rework (skeleton, dragging)

[robx]

Couple changes to make sukoro nicer:

• Visual mode where a skeleton is drawn behind numbers. Circles are replaced with just skeleton, and computed number of neighbours.
• Some support for dragging; dragging numbers always creates circles/skeleton.

Not quite done yet. Current state: https://garp.vllmrt.net/scratch/sukoro/p.html?sukoro/9/9/b1c3c1b2b4a1a1e3j3b1b1j2e4a2d1b2a1a3c2b

Todo:

• answer check should be ok with circles / skeleton only, no need to input all numbers
• mouse input is still a bit weird: right-button dragging of Xs; also apply to sukororoom
• get rid of keyboard player input / cursor entirely?
• when solving, it would be nice to be able to "lock in" an automatically computed value; perhaps give it a different color when all neighbour cells are marked?