Adding "back" button to Weather app (matching Agenda, Health, etc)?

Right, I can see why we want to avoid changing Bangle.setUI.

But as far as I can tell on Bangle.js 1:

    Bangle.setUI({mode:"clock",back:function() {
      Bangle.showClock();
    }});

opens the launcher (instead of the clock) when you press the middle button.

And

    Bangle.setUI({mode:"custom",back:function() {
      Bangle.showClock();
    }});

does nothing for button presses.

So we end up needing something like this:

    Bangle.setUI({mode:"custom",
        back: function() { 
            // Bangle.js 2 also calls this for button presses
            Bangle.showClock();
        },
        btn: global.BTN2 ? function(b) { 
            if (b==2) Bangle.showClock(); // only needed for Bangle.js 1
        } : undefined
    });

Bangle.js 2 setUI checks if there are any button listeners, and assigns the back function otherwise, Bangle.js 1 doesn't :-(

Maybe we could add similar code to the Bangle.js 1 setUI:

    Bangle.on("touch", touchHandler);
    if (Bangle.btnWatches===undefined) // only add back button handler if there are no existing watches
	    btnWatch = setWatch(function() {
	      btnWatch = undefined;
	      options.back();
	    }, BTN2, {edge:"falling"});
    ...
    // and in widget.remove:
    if (btnWatch) clearWatch(btnWatch);

But I suppose that would lead to the same problem of it not working in older firmware... (On the other hand: it doesn't work now either, so apps that already assign their own BTN2 listeners should be fine?)