Add application menu dbus paths to org_kde_plasma_window interface

Summary:
This patch adds an interface allowing a compositor to send
the service name and object path of a PlasmaWindow's application menu
to the client.

Reviewers: KWin, zzag, davidedmundson

Reviewed By: KWin, zzag, davidedmundson

Subscribers: davidedmundson, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27464