systemmonitor: Optimise source updates.

Summary:
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

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3141