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