UI: a delete that merely releases override is effectively not undoable #117
Description
Background: Deleting tree items that override inherited values releases the override, so the item now tracks the parent model. (A second delete on the inherited value causes the item to be removed entirely.) Undoing the delete restores the overridden value.
Problem: If the overridden value exactly matches the inherited value, then the undo has no effect, because it runs through a mechanism that removes such overrides. This is only a problem in a particular scenario where you create a child model, update the parent model to match the child, clear overrides in the child model, then decide to undo all of that. The child items continue to track the parent, rather then remaining at their intended values.
The solution is to modify all Delete* UI objects so they bypass the extra override-clearing step when undoing.
It seems unlikely that this is much of a problem in practice, so this is a low-priority issue to fix.