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

I don't really understand this. It appears that any transition from a non-null currentEditMode is treated as a cancel. So how do we switch modes in the normal apply case?  Could this be refactored into a separate cancelEditTool() function or something?  Note that with this refactor we never have to switch from one tool to another, only from a tool to no tool, so there is proabably more opportunity for refactoring here.

Or at least just add a longer comment explaining what's happening here.