taskmanager: performance

Summary:
Don't do queries on unknown values.
Don't create a KDesktopFile until we know we're going to need it

Test Plan: Ran plasma with debugs, didn't see any regressions.

Reviewers: #plasma, hein

Reviewed By: #plasma, hein

Subscribers: hein, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22162