Polygon: undo last step(s)

[MatthiasArnold]

the setting of points in a polygon should be un-doable. suggestion is to take back point by point, as for example the polygonal marquee function in image editing tools do (like photoshop).
so while the user is in polygon mode, all poiints should be tracked, so they can be un-done one by one, using, e.g. the key.

[MatthiasArnold]

cf #32