Add background portal

Summary:
Implements Background portal, providing APIS related to applications  running in the background.

Implemented according to: https://github.com/flatpak/xdg-desktop-portal/blob/master/data/org.freedesktop.impl.portal.Background.xml

Test Plan: Tested with manual invocations over DBus and checking what I return back through dbus-monitor.

Reviewers: #plasma, davidedmundson, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28214