Add an Undo option

[Durdge]

Particularly in advanced version, it would be useful to have an "undo" option that tracks the last 100+ changes. Change = blocks have changed position after a mouse-up even was triggered. i.e. dragging 5 blocks in a row over 20 columns will not create 20 undo events. It will create 1.

[waldispuhl]

The advanced version already has a mechanism to save arbitrary configurations (right column). It should be possible to implement an undo button relatively easily. I'll keep you updated here if we find the bandwidth to implement it in a near future.