Automatically set .json file ending for MxN layout files

Summary: When missing (which happens on Ubuntu, since the default suffix of QFileDialog does not seem to work there), the .json file ending is now automatically added for MxN layout files.

Test Plan: Open MxN, save the window layout. The created file should always be a .json file, regardless of if you added the extension explicitly or not.

Reviewers: O3 MITK Reviewer Group II, kislinsk

Reviewed By: O3 MITK Reviewer Group II, kislinsk

Maniphest Tasks: T29585

Differential Revision: https://phabricator.mitk.org/D830