Use tabs rather than spaces for the MuPDF project.
Fix disabled state of bar buttons on iPhone/iPod Touch.

Previously on iOS 7, bar buttons didn't change visually when disabled.

Also improves the look and feel - the buttons are now tinted in the
highlighted state rather than showing a grey background. On iOS 7, the
iPhone/iPad buttons style match.

Tested on iOS 6 iPad/iPod and iOS 7 iPad/iPod.