[LMP] Fix build.
[LMP] Refactored current track notifications for #1233.
[LMP] Added notifyCurrentTrack() slot for #1233.
[LMP] Shortcut for current track notification for #1233.
[LMP] Moved State enum out of SourceObject.
[LMP] Player::GetState().
[LMP] Properly reuse known info if possible for #1233.
[LMP] Emit songChanged({}) signal on state change only.
[LMP] Removed some debugging output.