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

UndoCropHandler is different from other aspect ratio buttons as it updates image to original size by resetting crop region and resetting preview.
 
https://github.com/mozilla-b2g/gaia/blob/master/apps/gallery/js/ImageEditor.js#L634

Also, UX is considering new UI treatment for this button and adding a new aspect ratio but that's still in discussion. I will also synch up with you on how to best handle Apply and Cancel when user presses return to original image. 

Should we display a message saying this will revert you image to original size and the changes can't be undone or cancels out and show previous cropped region.