Add a mode where all the bottom part can be hidden

Summary:
Referencing https://phabricator.kde.org/T7676

In HeaderBar, if playerControl.maximized == true, then we are in 'party
mode' otherwise we are in normal mode.

Reviewers: Elisa, januz, mgallien

Reviewed By: Elisa, mgallien

Subscribers: januz, mgallien, astippich

Differential Revision: https://phabricator.kde.org/D14736