[System Tray] Unified data model for System Tray items

Summary:
Currently there are two different sources of SystemTray items:
* Plasmoids
* Status Notifier

Thi change adds new model that holds both Plasmoids and SNIs.

Test Plan: Add/disable applets. Start/stop SNI app.

Reviewers: #plasma, broulik, ngraham, nicolasfella

Reviewed By: nicolasfella

Subscribers: plasma-devel, nicolasfella, anthonyfieroni

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25580