Minor tweaks

A few small things I changed when adding undoable to our project:

- Hiding the target after calling showUndo allows the callback to investigate
  the normal .width() of the target and set the undoable width appropriately,
  making it possible to achieve the one open issue about making the
  undoable row same size as the original content.

- a target with no class gets the literal class "undefined" in the undoable,
  now it watches out for that and uses an empty string instead.