Set default visibility for per-process network sensors to true

Summary:
Since it is a nice feature that provides useful information and should
not be hidden.

Depends on D24053

Test Plan:
Start ksysguard ensuring a clean config, observe that the Download and Upload
columns are visible.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24054