appmenu: new protocol

This merge request adds a new appmenu protocol, allowing clients to notify compositors about the DBus object path and service name of their application menu implementing the com.canonical.dbusmenu interface. This allows a compositor to render application menus using its own UI or forwarding this information to a client dedicated to rendering application menus.

This protocol is based off of the org_kde_kwin_appmenu protocol implemented by KDE's KWin compositor and plasma-integration for Qt clients.