Simple Text-Only Canvas

[tomcrane]

This is an issue to capture the Delft exhibition special case of an "info" panel Canvas.

Here, the Canvas is 1000 x 1000, it has a single painting annotation with an HTML TextualBody, that the exhibition environment renders as the info panel, and a single describing annotation with an HTML body that the exhibition environment opens as a modal popup from a "Read More" link.

There are examples in all the exhibitions. Canvases have the behavior property "info" (as well as layout behaviors).

At this stage we don't need to solve the general problem of mixing textual annotations with images and AV. Like #226, this is a special simplified case - we need to be able to open them, edit them, create them.

All the parts for this should be present already, just need to determine how the Manifest Editor presents these canvases.

All the manifests in https://digirati-co-uk.github.io/delft-exhibition-converter/ (Converted) demonstrate these info panels.