Fix log playback GUI display (backport #2611)

[mergify]

🦟 Bug fix

Fixes gazebosim/gazebo_test_cases#1640

Alternative to: #2590

Summary

Loading the logging playback plugin before SceneBroadcaster prevents playback from being displayed on the GUI. This order was changed in #2452 which caused the regression.

I haven't had time to delve into why the order matters, but it can easily be verified by running test/worlds/log_playback.sdf with <playback_path> set to the log directory. If the plugin comes before SceneBroadcaster, the playback is not displayed on the GUI.

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

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

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This is an automatic backport of pull request #2611 done by [Mergify](https://mergify.com).

[mergify]

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