* add SubsonicScrobbler, add Scrobble method in SubsonicService
* new class SubsonicScrobbleRequest, use queue again, clean up
* add checkbox to enable server-side scrobbling to Subsonic settings page
* Check serversidescrobbling in SubsonicScrobbler::ReloadSettings instead of SubsonicService
TODO: SubsonicScrobbler::ReloadSettings needs to be called when
SubsonicSettings change.