Fix Transport Plotting GUI plugin (backport #667)

[mergify]

🦟 Bug fix

Summary

The transport gui plugin tool was broken by #583 as it mistakenly removed the PlottingIface object that's needed for QML -> C++ integration to work. This PR adds the variable back in the private implementation class.

Without the changes in this PR, when plotting a field from a topic, you'll see the following warning message in the console:

[warning] [Application.cc:915] [GUI] [QT] qrc:/qml/PlottingInterface.qml:114: ReferenceError: PlottingIface is not defined

To test

1. Open Transport Plotting GUI plugin
2. Open Topic Viewer plugin
3. Try plotting any field from a topic
4. Hit Play -> it should work now and you should see a line drawn on the graph

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 #667 done by [Mergify](https://mergify.com).