Add small app for testing appmenu

Summary:
App which exports a menu, designed to simulate so that the dbus menu can
be easily extended for obscure conditions then tested and debugged

Test Plan: Exports a menu

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4058