Make use of notify_main_thread callback

The sp_session_callbacks::notify_main_thread callback does not work as expected, and creates a crash when used.

The timeout value returned by sp_session_process_events(session, timeout) doesn't seem to work as documented:
https://developer.spotify.com/en/libspotify/docs/group__session.html#g019a640b351de2a1ffe040ed755013f4

Both of these issues need to be addressed, as currently this timeout value is ignored and the fixed QThread::usleep(10000) is used instead.