Double click detail editing for polygons

As discussed in https://github.com/godotengine/godot/pull/17112: allow polygon vertices to be edited using a special edit mode reached through double clicking on a polygon. With a simple click, only the selection rect is available. This fixes the problem of confusing selection rect and polygon vertex markers.

This implementation is pretty hacky and I'm sure @groud will not want to see this in master, which is quite ok. Still, this PR can serve as a place of discussion how this double clicking editing mode could look like in detail.

Demo of the current PR here:

https://www.dropbox.com/s/mmqluq3t7820k7g/poly-edit-rect-test-1.mp4?dl=0