Minor cleanups of TabSwitcher plugin

Summary:
Changes include:
- fix one memory leak
- use KTextEditor::Document in model data
- avoid using QStandardItem, since no standard models are used

This is a follow-up change to 47cf54dd

Test Plan: manual testing

Reviewers: gregormi

Reviewed By: gregormi

Subscribers: kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D16341