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