messagegui: refactor the messages scroller and use it more (WIP)

[thyttan]

WIP

Initializes the scroller with three messages loaded. When scrolling to the top/bottom new messages are loaded.

This is far from merge ready and I'm not sure it should ever be merged. Should be implemented as a setting if to be merged I think.

With that said it can be tried from my app loader: https://thyttan.github.io/BangleApps/?id=messagegui

[thyttan]

I'm testing/developing this by generating test messages from the WebIDE console field by pasting:

let i = 0;
let msgInterval = setInterval(
  ()=>{  
    print(i);
    GB({"t":"notify","id":1575479849+i,"src":"Hangouts","sender":"sender"+i,"title":"A Name"+i,"body":"message contents that are very similar to one another. in that they are somewhat long, and exactly the same. Let's have it go on and on and so forth for a while to make sure we overflow properly."});
    i++;
    if (i == 9) {clearInterval(msgInterval); delete i; delete msgInterval;}
  }, 500
);

[thyttan]

@bobrippling If you'd help me by testing this a bit I'd be interested to hear your thoughts!