Add ability to undo work.

The common use case for me is that I perform a scan on a group of files, and the search returns incorrect albums for all songs in the right hand pane.  If all of the songs in the album in the left hand pane have been found (even incorrectly), the album goes away and it's no longer possible to restore them in order to perform work manually.  The most expedient thing to do at that point is to exit and restart Picard.

Also, if I edit a tag incorrectly and want to undo the edit.