New header style for playlist

Summary:
This patch moves the buttons in the playlist to the top and groups them. I also changed the clear playlist icon to the trash icon (I think it conveys the meaning better), and also tweaked the buttons text for more consistency.

Reviewers: Elisa, mgallien, astippich

Reviewed By: Elisa, astippich

Subscribers: mgallien, astippich, ngraham

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D14015