fix: don't cache icon background

[akloeckner]

fixes #425

[rchl]

Actually, I've just realized some potential issues with it. The background of the entity can change dynamically - for example, media player tiles' background should change on every song. I think this will break that.

[akloeckner]

That's true. But it was broken before then already. Nice thing about the new implementation with cacheInItem is that we can simply remove that function call and be good... ;-)

Shall we? I don't know the inner workings of AngularJS well enough.

We could also add a dummy cache, which is updated constantly, but will prevent any watchers from executing. Or do we need the watchers to execute in order to update the background? Seems a bit overdone to me, anyways.

[rchl]

It wasn't broken before. Probably because the entity was recreated every time there was an update so caching didn't really do much in practice.

If we return a new object every time this function is called then angular will notify all watchers, even if the values didn't really change. And that means that the DOM style property will be updated every time. I guess that was happening before anyway and the browser will actually optimize it so that nothing really happens. So while ideally we would not notify listeners, I'm fine with just returning a new object every time.