Bangle_setUI_Q3: fix btn back handler was not always rm:ed correctly

[thyttan]

Using the recent messagegui updates in combination with Fastload Utils exposed a bug with Bangle.setUI where back on hw button watches could end up not cleared after setting up a the UI of the clock after unreadTimeout of messagegui ran out. This seems to fix that, when testing this special case at least. I just finished the changes so have not tested for long.

@gfwilliams please say if we should change something before merge.

When this looks good I can fix the Bangle.js 1 implementation as well.

[gfwilliams]

Sorry - missed this last week!

[gfwilliams]

I've just committed some changes - I think we don't need to worry about removing the listener in .remove since the only place we call it is setUI line 11, and we clear the button watches 2 lines later.

It makes the code much tidier - and it's probably something I should have done originally. Does that look good?

If so we can do the same on Bangle.js 1?

[thyttan]

I think that will do :) let's go with that.

I will probably not tweak Bangle 1 setUI until next week. Please go ahead if you want to do it instead!