Centralize interaction with notifications

Summary:
Make sure clicking on them will work the same on the popup and the delegate,
there were some issues on one or the other, now all the code is centralized
in NotificationItem.

Test Plan: Been testing locally, repeatedly

Reviewers: Plasma, davidedmundson, mart, albertvaka

Reviewed By: mart, albertvaka

Subscribers: mck182, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4545