Close KWordQuiz documents before closing the app

Summary: The current KWordQuiz fails to close its documents and thus to remove the lock in ~/.local/share/stalefiles/kwordquiz. This issue prevents them to be saved after the next edits (KwordQuiz reports that they are in use by some other application). Moreover, this (and some other things that I could not detect) leads to KwordQuiz crash every time when you close the documents.

Test Plan:
1. Open KWordQuiz.
2. Type something.
3. Press Ctrl+Q.
4. Save document somewhere.
5. Open KwordQuiz and use "File->Recent documents" to open the saved document again.
6. Type something.
7. Press Ctrl+Q.
8. Confirm saving the document.
9. The document should be saved without warnings.

Reviewers: #kde_edu, aacid

Reviewed By: aacid

Subscribers: aacid, apol, kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D22299