Bug 798696 - Gallery back button, move the button to a header element

Fix for [#798696](https://bugzilla.mozilla.org/show_bug.cgi?id=798696). At the moment it is not clear how to get back to the photo list from the photo detail page. The button has a different icon than normally and is at a different place. Also all the icons in the footer are used to do an *action* on the current image, and the back button should have no place there.

This patch is a proposal on how to fix this. I have moved the back button to a 0.75 opacity dark header element (that goes away if you tap the photo). It looks pretty good actually, and it is way more clear on how to go back.