Delete board positions that contain no move without discarding future board positions

[herzbube]

Currently the user cannot delete a board position that contains no move without also having to discard all future board positions. This restriction should be removed, because most board positions that contain no move do not have state, so deleting them does not affect future board positions.

The only known exception to this at the moment is board positions that contain the SGF property DD. When implementing this feature all SGF properties should be reviewed in regard to their stateful-ness.

In case variations are already implemented, also consider that deleting a board position that is at a variation branching point probably needs special handling.