Issue 537 UI widgets

[bergos]

This PR adds the widgets from datashapes.org. The namespace was adapted, and other minor changes were made to align it with the other SHACL specifications.

The selection of widgets and the details in the widget should be handled in a separate PR. This is an initial starting point so that other PRs can refer to widgets.

dash:applicableToClass and dash:rootClass was kept as there is no matching SHACL property. The dash namespaces should be easier to notice when we work on the details.

Closes #537

• See this document rendered online here

[TallTed]

Note that accessibility review will probably advise us to replace png with svg wherever possible, and to include textual descriptions of the images, both to accommodate various visual impairments and common text-to-speech tools, among others.

[edmondchuc]

Looks good, just made some small suggestions to use the respec css class for examples.

[TallTed]

Line breaks in the ReSpec source (aside from those that are about keeping the lines below some maximum width) that are manually inserted to help comprehension of that source are usually if not always worth propagating to the rendered .html (via <br>).

[afs]

Note that accessibility review will probably advise us to replace png with svg wherever possible

Another point in favour of SVG is maintainability. SVG is a portable, editable format. PNG is not editable. The downside is the possible extra initial work for SVG.