Reduce interface stuttering
This reduces interface stuttering when playing new tracks or updating the collection.
I have about 9000 tracks on a NFS drive. Previously updating that folder let Amarok's interface hang for about 20 seconds. Now it just stutters a little bit.
This further improves the situation, because it makes the loading of lyrics async.
Reviewers: Amarok, markey
Reviewed By: Amarok, markey
Differential Revision: https://phabricator.kde.org/D11868