Create a Bangle.isWorn() method

Temperature feels really tricky, I can easily imagine it getting higher from e.g. leaving the watch in the sun than wearing it.

Quiet mode has 3 levels:
0 = not quiet
1 = only alarms
2 = totally quiet
Maybe including this in the Quiet Mode Schedule app would be a good idea? (it looks like it could do with an update to the new sched library anyway...)

Instead of a isWorn() function, could it maybe work with events and a global var?
Basically add a boot app that keeps track of movement/charging/etc, and does something like

(function(){
      function CheckWearing(event) {
        if (/* watch is being worn */) {
            Bangle.isWorn = true;
            Bangle.emit('worn', true);
        } else {
            Bangle.isWorn = false;
            Bangle.emit('worn', false);
        }
      }
      Bangle.on('hrm', checkWearing);
      Bangle.on('accell', checkWearing);
})()

That way other apps can check Bangle.isWorn !== false and/or do Bangle.on('worn', wearing => {});, without needing to include e.g. a library in every widget.