PolyPen: Always knife when hovering over existing geometry

[jlampel]

(copied from #1407 (comment))

Switching to a knife action whenever hovering over existing geo would be an awesome improvement. In retopology, we can assume that the user does not want to create overlapping geometry.

The open question is where the cut should start from, but I think you could just use the center of the selection.

For example, this:

Would be like this (except the selection would remain):

And this:

Would result in this (except the selection would remain):