CoverCache - emitting "requestRepaint()" when the caller is CoverDelegate + connect this signal to WTrackTableView::update()

It fix some issues with the traktor_view:
@kain88-de
- The covers don't get painted once they are loaded, you need to scroll again to issue another paint event
- If you click a row in the libraryview the cover is first overlayed with blue and after a second it shows the normal cover again while the rest of the row is still blue