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 47cf54ddf423f4765b406ebe3e0cb8dcc5ca52a2
Test Plan: manual testing
Reviewers: gregormi
Reviewed By: gregormi
Subscribers: kwrite-devel
Tags: Kate
Differential Revision: https://phabricator.kde.org/D16341