[KCMs/Notifications] Tweak layout and strings to have a bit more visual grouping for the logical sections
Test Plan:
Reviewers: broulik, Plasma, VDG, ndavis
Reviewed By: broulik, Plasma, VDG, ndavis
Subscribers: ndavis, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D26379