[Notifications KCM] Use nice new bell-style notification icon
Summary: We have a nice new icon, added by @trickyricky26 in D18533. Let's us it!
Test Plan:
Reviewers: VDG, ndavis
Reviewed By: VDG, ndavis
Subscribers: trickyricky26, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D18678