Add explicit AppMenu protocol
Summary:
A protocol that attaches to a surface and contains two strings which can
change.
The intended use is for clients to link a DBus Appmenu object with a
surface.
This is in preparation for the Qt Extended Surface deprecation which
currently handles this in Kwin.
Test Plan: Attached unit test
Reviewers: Plasma, graesslin
Reviewed By: Plasma, graesslin
Subscribers: broulik, graesslin, plasma-devel, Frameworks
Tags: Frameworks, Plasma on Wayland
Differential Revision: https://phabricator.kde.org/D8919