[Issue] Undo doesn't remove UserText attributes

[Dancergraham (Graham Knapp)]

@clement previously gave me this solution under a different username (I forgot I had that account!)

Remembering values : problem with undo

@Graham_Knapp, Rhino allows to setup custom undo events. There is a good example dealing undoing sticky values here. _ c.

Looks like I need to be brave and implement that one, better late than never