Bug 1031890 - Vertical homescreen takes a long time to load

At this point, if we do
```js
setTimeout(this.doRenderIcon.bind(this, icon), 2000);
```
Icons are displayed faster because we had time to paint the cached icons here
style.backgroundImage = 'url(' + URL.createObjectURL(blob) + ')';
Without timeout, cached icons are slower because the fetching process starts while gecko is trying to paint them