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

Again, I think this utility function would be better off at the end, so it doesn't distract from the real work of the function.  Also, however, since you only use it once, you could consider getting rid of the function entirely and using the code inline below.

Or make a general-purpose showBanner(bannerId, messageId) function that you can use both in gallery.js and in ImageEditor.js. You could even put that it its own show_banner.js module even though it would be very short.