[libtaskmanager] Add ApplicationMenu{ObjectPath,ServiceName} roles to model

Summary:
The tasks model now exposes a window's application menu object path and service name.

No update signals set up for XOrg; not sure what to listen for.

Test Plan:
See that the model has more roles and see that they're wired to the application
menu obiect path and service name on Wayland and XOrg.

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27959