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