fix music not being stopped when clearing the play list

Summary: fix music not being stopped when clearing the play list

Test Plan:
when playlist plays and we press the clear button on playlist, play stop and
if we enqueue new tracks, play does not start

Reviewers: Elisa, astippich

Reviewed By: Elisa, astippich

Subscribers: astippich

Differential Revision: https://phabricator.kde.org/D11159