Add optimisation in CollectionModel class.

Check if “artist“, “query“ and “search“ really changed before emiting a
signal. We win some precious time when populating the model is slow.
Minor modifications in QML Collection component.
Yellow highlight in QML CollectionView component.

The delegate's background is forced to be transparent if current item.
The highlight is a light yellow rectangle. The horizontal separator
before the current item is set to a darker yellow color (shadow).
When the model emits the "modelReset“ signal, we set the view's current
index to -1. Add active focus trough mouse click in the Collection
component. Not sure if this is done right. Would be nice to have a nice
focus chain on <tab>.