gzclient: improve startup reliability

[scpeters]

🦟 Bug fix

This is an additional fix for https://github.com/gazebosim/gazebo-classic/issues/681, following up from #3121

Summary

In #3121, a call to the /scene_info service was added in the Scene class to improve the reliability of initializing gzclient. This improved reliability, but I have still observed flaky gzclient startups, particularly when loading worlds with large Digital Elevation Maps (DEMs). I found another place where ~/request and ~/response topics are used to get scene info in the MainWindow class, which I suspect is an additional source of unreliable start-up behavior.

This applies similar changes from #3121 to the MainWindow class by getting scene info from the /scene_info service and falling back to using the request / response topics if the service is not available or the service call fails.

Checklist

• [X] 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

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