Rework shuffle handling

Summary:
Remove deprecated usage of qrand().
Keep a list of the previous, current and next track
when in shuffle mode, to be used in a following patch.
Simplify calling skip and previousTracks

Depends on D22769

Test Plan: all tests pass

Reviewers: ngraham, mgallien

Reviewed By: mgallien

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D22770