Give the GHNS button an appropriate icon and more consistent text

Summary:
This patch gives the GHNS button an icon and tweaks its text to be consistent with the general pattern seen in other KCMs.
BUG: 400424
FIXED-IN: 5.15.0

Test Plan: {F6371684}

Reviewers: #plasma, #vdg, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16497