New plugin type: rendering ScenePlugin

[osrf-migration]

Original report (archived issue) by Louise Poubel (Bitbucket: chapulina, GitHub: chapulina).

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On Gazebo rendering, there's already a VisualPlugin, but it would be also nice to have a plugin type in rendering which is tied to the rendering::Scene itself and not a single visual. It would be analogous to how we have ModelPlugins and WorldPlugins in physics.

The plugin introduced on pull request #2916 for example could be a scene plugin.