Feature/shelli ui

@MartijnBraam not sure how to proceed here. This is a MR to the postmarketos branch feature/shelli-ui, so maybe you can merge it in and we can proceed with the code review?

Have released both gesture and shelli and tested the pmaports changes locally building and on nexus 5.

The power key works fine for me and the freeze after sitting for about 5 minutes still happens for me but I don't consider that necessarily a shelli bug, maybe the code needs to feed a watchdog or something.