Swipe between sheet music and lyrics

[ztrupert]

A new navigation option in addition to or in place of the current sheet music button. The user could swipe horizontally on their screen to move quickly back and forth between the sheet music and simplified lyric pages.

[thebiblelover7]

This would conflict with issue #2, so we'll have to decide on which one. I'd rather have the swipe to switch hymns than sheet music to be honest, but I'll think more about it.