Bug 1042319 - [Gallery] Update gallery edit UI r=djf

It would be nice to do this without introducing a new global...  ImageEditor.lastEditedFile or something. You could set it to the filename of the file that was just saved.  But I'm not sure that is a lot better than just using a global.

This is such a special-purpose global variable, used so infrequently that I'd at least give it a more verbose name like justSavedEditedImage or something like that.