Implement needs_background metadata

[martinRenou]

Describe the issue

The matplotlib-inline backend provides the needs_background metadata which allows to force a light background under plot images to improve readability when using a JupyterLab dark theme. We should probably implement the same for the static image.

See https://github.com/ipython/matplotlib-inline/blob/541b26da2f49d0403994b91cb062e8c438920c44/matplotlib_inline/backend_inline.py#L226-L239

This will not work for the widget repr, but we could probably implement custom logic to mimick this behavior with the widget view.