Improve octave login

Summary: Before, when we work with octave and use 'Restart Backend' button, we suddenly get foreign output. So this commit fix it by adding backend cleanup of inner state in logout.

Test Plan:

1. Run octave backend without patch
2. Write some entries and run they
3. Reboot octave backend by the button and check, that the output changes
4. Apply patch
5. Check, that now all fine.

Reviewers: Cantor, asemke

Reviewed By: Cantor, asemke

Subscribers: Cantor, KDE Edu

Tags: KDE Edu, Cantor

Differential Revision: https://phabricator.kde.org/D12565