Move some Magnatune specific stuff out of the ServiceSqlcollection and into the new MagnatuneSqlCollection sub class.

Also, make trackForUrl work in a sane way with redirected magnatune membership urls by using the db to fill in as much information as possible but maintain the original url (this way, it will work if you are logged into the Magnatune site but does not have Amarok configured with your membership information. Useful, for instance, when demostrating the Magnatune membership integration to non members on their Amarok, or if you prefer to browse the site and just use Amarok for playback).