Delete shadows together with stones

This resolves the problem that shadows of handicap stones were drawn at the wrong position.

Handicap stones are apparently created twice when loading an SGF, the first instance being immediately deleted.
This is not desirable, but I do not understand why exactly this happens.
Fix handicap stone handling in SGF files

Previously it was assumed that handicap placement is always fixed
and handicap stones were placed according to HA[] field.
Handicap stones' position should be declared with AB[] instead.