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