Get .desktop file name for launcher URL and activities string from KWindowInfo.

Summary:
The former depends on https://git.reviewboard.kde.org/r/129327/

The latter is a thematically related cleanup I did along the way.
The strange original code was probably to handle the NULL_UUID, but
TaskFilterProxyModel (where this data role becomes relevant)
developed the ability to do this since.

Reviewers: Plasma, graesslin

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3262