[applets/SystemTray] Implement sorting in the model

Summary:
Replace two wrapping PlasmaCore.SortFilterModel with custom sorting on model side. Extract common code to base class.
This fixes small issue in sorting and allows to implement other improvements (later on).

Test Plan: Everything should work as before.

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

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

Subscribers: mart, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27088