Undo for when user clicks in an image accidentally

[Elizabeth Brainerd]

This may be hard to do, but I find that I often click accidentally in an image with a point selected and it moves to the wrong place. Would be nice to be able to undo that click.

[Benjamin Knorlein]

fixes #65 : Added an undo feature (Ctrl-Z) as long as the active point, camera and trial does not change

→ <<cset 3e9870a55012>>