Fix FileDialog when saving world

[iche033]

🦟 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)
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• All tests passed (See test coverage)
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[iche033]

@Mergifyio backport gz-sim10

[mergify]

backport gz-sim10

✅ Backports have been created

• #3112 Fix FileDialog when saving world (backport #3110) has been created for branch gz-sim10