Undo/redo drawing points

[mstenta]

It might be useful to add simple "Undo" and "Redo" buttons while you are drawing points/lines/polygons, which simply undo/redo the last point(s) you added to a shape.

This would be especially useful if we do #51 - because GPS points can be pretty inaccurate at times, so you might need to undo dropped points.