Topic Viewer doesn't display vector types

[apojomovsky]

The Topic Viewer widget is unable to show the payload of the vector-typed fields contained in an ignition message.
An example is shown below, where the messages /agents/state and /visualizer/scene_update have, apart from the header field, vector-typed fiels that are not currently displayed.

[hidmic]

@stonier @apojomovsky Contrary to what I would've thought, lack of support for vector types may have been a design choice. Compare TopicViewer's implementation, the one whose render is shown above, with TopicInterface's rendering widget implementation, which we use for the SceneTree view. Actually, the method in TopicViewer that does all message parsing states that only plottable fields are shown.

What about adding another TopicInterface for agents' states then? Seems like an easy solution. Though I'm sure @caguero or @nkoenig will know better than me.