Allow playlist to be hidden

Summary:
This patch adds an option to the main menu to hide/show the side playlist.
This was an often requested feature (and it also looks kind of cool).

Feature: 392781

Reviewers: mgallien, astippich

Reviewed By: mgallien

Subscribers: ngraham

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D13834