Use new Notifications KCM icon in other appropriate places too

Summary: D18678 changes the Notifications KCM's icon. This patch uses that same icon in the Notifications widget in places where clicking on it will open the KCM.

Test Plan: {F6586488, size=full}

Reviewers: broulik, #plasma

Reviewed By: broulik, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18723