Add item speedup

[vrutkovs]

Make item loading nice:

• Faster item fill in albums / songs view
• Crossfade transition on artists view to hide elements loading

[kyoushuu]

The adding of the items did get faster, but the loading of the media art is now slower. But the code for lookup will be removed anyway in the new future, which what causes the slowness. So I think this is good for merge.