Rank Kickoff desktopsessions search results over services

Summary:
BUG: 405268

Users more frequently want to log out rather than change desktop session settings. Also makes it consistent with KRunner.

Test Plan: Open Kickoff and search for "logout".

Reviewers: Plasma, VDG, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20923