[System Tray] Clean up popup code

Summary:
This removes hacks and does things in a cleaner way.

Credits go to @broulik.

Reviewers: #plasma, broulik, ngraham

Reviewed By: ngraham

Subscribers: ngraham, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25614