Fix for #3860 (undo bug on create way)

[stevage]

This fixes the funniness around undo/redo for creating new ways:

"Click two or more points (=n) on map - a way connecting clicked points is being created. After (n-1) Undo's the way is gone, but the initial point still remains visible (a green dot) and the Undo button is already grayed out - one step got forgot.

It is possible to delete the point manually (red (x) button), but it should be also deleted by the n-th Undo."

My head hurts now. In an ideal world I would have squashed the three commits into one. I tried that, but Git growled at me and threatened to take my work and never give it back, so we compromised.

[systemed]

I have a branch to test this and looking at it properly is my next todo. :)