[SystemTray] Use unified data model everywhere

Summary:
Use the unified data model everywhere, not just in configuration. This simplifies UI code, separetes presentation from data.
This requires the use of ListView and GridView.
This change enables the implementation of more advanced sorting algorithms for systemtray items.

Test Plan: Affects almost all areas of system tray.

Reviewers: #plasma_workspaces, #plasma, davidedmundson, ngraham, broulik

Reviewed By: #plasma_workspaces, #plasma, davidedmundson, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26992