rose edit: fix change variable added by "revert page"

Currently, altering the value of a variable that was added by "revert page" (i.e. it had been removed since the last save) would not produce modification icons (although the value was in fact changed properly). This is because the added variable is a direct reference to the "last save" reference variable, and changing the value in the added variable also changes the value of the reference variable - hence, no changes flagged since the last save.

This change makes the added variable a copy of the "last save" reference variable, so this cannot happen.

The first commit is a tidy of a widget change (has little practical effect, but is safer).

@matthewrmshin, please review.