Add footer support to systray

Summary: This adds a footer element that will replace with a possible footer that the contained plasmoid could have, given that they are using a Page.

Test Plan:

Reviewers: ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T10470

Differential Revision: https://phabricator.kde.org/D29142