[KCMs/Notifications] Tweak layout and strings to have a bit more visual...

[KCMs/Notifications] Tweak layout and strings to have a bit more visual grouping for the logical sections

Test Plan: {F7858028}

Reviewers: broulik, #plasma, #vdg, ndavis

Reviewed By: broulik, #plasma, #vdg, ndavis

Subscribers: ndavis, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26379