Cleanup, configurable top and bottom bars

I've done a cleanup pass through the code, and then went ahead and added config options to place caption, close and nav controls in either the top or bottom bars. Defaults are as previous (caption in top, close + nav in bottom).

A minor thing I did was add a bit of transparency to the backdrop, so that users get less confused about whether this is a new page. Pressing 'back' in history does not have the desired effect for the use case - it will step to the last history page rather than closing the viewer.