Update last.fm login mechanism to use non-deprecated token method & add more error handling

The mobile session seems to be somewhat deprecated, and at least I'm not totally confident that it is actually possible to log in new sessions with it any more in Amarok.

Token-based method doesn't need password, so no new data is saved to kwallet in any case. Tries to use existing session key from config file and username from config file or kwallet. If username is saved to kwallet, tries to migrate it from there. If this succeeds, everything should be transparent to user. If not, an error window is shown and the account can be reconnected in plugin settings.

Also avoids

BUG: 414826

Additionally, add some more error handling; specifically InvalidSessionKey

BUG: 327547