New Bangle.js 2 menu system

@ChrisS thanks - I'll look into this now.

I'm not entirely sure we want to "sacrifice" swiping right on every screen with a back button though

Yeah, although I'm struggling to think of screens where there might be a back button and you'd use swiping? Usually it'd be in some kind of menu, and it's the main app where you'd use swiping?