Summary:
Rework the mediaplayercontrol to use the new previous
and next tracks from mediaplaylist. Allows a large
cleanup.

Depends on D22771

Test Plan:
tests pass
play, next and previous buttons are still enabled as before
Random and repeat buttons still work as before

Reviewers: ngraham, mgallien

Reviewed By: mgallien

Tags: #elisa

Differential Revision: https://phabricator.kde.org/D22772