Add Extension to Filename to Save to Kig Format

Summary:
If the user chose to save in Kig format, then just add the extension
as there is no other way to save it.

CCBUG: 404366

Test Plan:

1. Save without extension (might need to disable the "Add .kig extension" box).
2. In the error dialog, select "Save Kig Format".
3. Filename should now have the .kig extension.
4. Repeat, but this time trying to save with a different extension than .kig.

Reviewers: KDE Edu, yurchor

Reviewed By: yurchor

Subscribers: yurchor, kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D26165