Context menu for statusbar

Summary:
This adds a simple context menu to the statusbar;
the only option it has is "Hide", which obviously hides
the statusbar the same way as if you were to go the view menu.

Reviewers: drosca

Reviewed By: drosca

Subscribers: drosca, falkon

Tags: Falkon

Differential Revision: https://phabricator.kde.org/D16158