Debug layer to show node movement

[joshdoe]

I just thought of this after commenting on #7295; it would be useful to have a debug layer that shows node movements from the last (or configurable) command in the stack. As in the conflation plugin, the layer can simply show arrows indicating the movement from the old to the new position after performing an action like Replace Geometry. This could perhaps be expanded to show other changes I can't think of right now.

As for implementing, I'm not sure the best way; perhaps have a duplicate DataSet that's always one command behind (which might be too computationally intensive to find all changes), or just analyze the commands themselves for the changes.