Make MediaPlayList responsible for remaining tracks
Summary:
Make MediaPlayList responsible for remaining tracks.
Allows simplification by only reacting to currentTrack
changes inside manageheaderbar.
Add tests for remainingTracks and cleanup manageheaderbar.
Fixes the remaining tracks showing when in playlist repeat
or shuffle mode.
Test Plan:
tests pass
Remaining tracks are only showed for when not in
shuffle or repeat mode
Reviewers: ngraham, mgallien
Reviewed By: mgallien
Tags: Elisa
Differential Revision: https://phabricator.kde.org/D22769