Export full visualisations as images (with axes, title, legend, color bar, etc.)

[axelboc]

It should be possible to export visualisations as PNG images, just as they are displayed in the viewer - i.e. with axes, axis labels, title, color bar, zoom level, etc. but also with some extra features as required (e.g. a legend in the corner for line visualisations with auxiliaries, as per https://github.com/silx-kit/h5web/issues/970#issuecomment-1085720334)