Allow Decrease Step without selection

[manongjohn]

This relaxes the requirement for Decrease Step where you need to select multiple cells in 1 or more levels.

Now you only need to be on 1 cell or 1 single across multiple levels and decrease step will continue to reduce held exposures until a different drawing is encountered. Selecting multiple cells will still work as usual.

This will allow users to use Increase/Decrease Step to adjust timing of exposures at single level, rather than having to lock column and use Insert/Remove Frame. Does not shift keyframes though (future enhancement?)

Also corrects a bug where if you use Decrease Step on empty cells but then Undo, it will shift cells down.