Only show system total cpu usage by default on high core count machines

Summary:
Currently, the default configuration of showing all individual cores is pretty useless on system with a high number of cores.

This changes the default to only show all individual cores on systems with 8 cores or less, on systems with more cores only the total cpu usage is shown

Reviewers: #plasma, #vdg, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: ngraham, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21690