Reintroducing Last.fm Similar Artists applet for Amarok 3

The Amarok 2 backend somewhat updated / ported away from Plasma DataEngine structures + turned into QStandardItemModel. New QML UI implemented.

Last.fm API doesn't provide artist images any more, so the applet searches local collection for albums by the selected artist and displays an album cover if found. Additionally clicking the album opens search for the artist in collection browser.