Remember expanded status of tree view in statistics main window

Summary:
The collapsed tree view items of LessonStatisticsView were saved in
parleys config file. With this stored lesson names the expanded status
of the tree items will be restored when StatisticsMainWindow is
displayed next time.

Reviewers: #kde_edu, aacid

Reviewed By: aacid

Subscribers: aacid

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D9931