systemmonitor: Optimise source updates.
Profiling indicates the highest CPU usage process during idle is the
system monitor engine. This attempts to get rid of the most obvious
usage, which was QStringList::indexOf, and QString temporaries.
Test Plan: Profiling only, as well as no local functional regression.
Reviewers: apol, Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Differential Revision: https://phabricator.kde.org/D3141