Port Render event from ign-gazebo

[chapulina]

The Render event was added to ign-gazebo so that various GUI plugins could run code that was guaranteed to run on ign-rendering's main thread. This is important when making changes to the rendering scene.

This concept would be very helpful to have in ign-gui so that we can write pure ign-gui plugins that affect the rendering scene. This should allow us to port a few plugins that are currently in ign-gazebo but could be here, like the GridConfig.

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The way the GUI events are working on ign-gazebo is a bit involved:

1. The Scene3D plugin emits the Render event from the rendering thread to the main window.
2. Other plugins can filter all the main window events and do what they need when they see a render event.

This is a bit involved and maybe even hacky, but it accomplishes cross-plugin communication without direct dependencies between them, on the same thread and without polluting the transport space.