Fix FileDialog when saving world (backport #3110)

[mergify]

🦟 Bug fix

Fixes #3109

Summary

Currently with the FileDialog.OpenFile file mode, users can only save the world sdf to an existing file. This PR changes the file mode to FileDilaog.SaveFile so that users can create a new file to save.

This textfield for naming a new file should now be available in the file dialog. On ubuntu, this will appear at the top of the dialog:

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers
• Was GenAI used to generate this PR? If so, make sure to add "Generated-by" to your commits. (See this policy for more info.)

Generated-by: Remove this if GenAI was not used.

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by and Generated-by messages.

---

This is an automatic backport of pull request #3110 done by [Mergify](https://mergify.com).