Add support for flatpak portals

Summary:
Creates a new flatpak notification plugin, which just forwards messages through
DBus to xdg-desktop-portal service, otherwise there is no way for sandboxed applications
to send notifications because of limited access. This plugin is used instead of
notify by popup plugin if we detect we are in sandbox and the xdg-desktop-portal
service is running

Reviewers: mck182, broulik

Reviewed By: broulik

Subscribers: broulik, apol, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D4425