Simple "YouTube" canvas

[tomcrane]

This is designed to accommodate the special case seen in Delft Exhibitions where a Canvas carries a YouTube video.

It's not the general AV support use case (that can come later).

For this special kind of exhibition Canvas, only one piece of data is needed - the Youtube embded URL (which may have a start time).

Example:

                            "type": "Annotation",
                            "body": {
                                "id": "https://www.youtube.com/watch?v=popa6e9Sv5E",
                                "type": "Video",
                                "service": [
                                    {
                                        "profile": "http://digirati.com/objectifier",
                                        "params": {
                                            "data": "https://www.youtube.com/embed/popa6e9Sv5E?start=89"
                                        }
                                    }
                                ]
                            }

(this is using the object tag model, we can switch - see https://github.com/digirati-co-uk/delft-exhibition-converter/blob/11a9c713a25862f5c4f0c8a99654b2d8d9b40370/main.py#L400)

The difficulty in this task is not allowing such a canvas to be created or edited in the manifest editor - it only needs one piece of data and we don't even need to render the video in the editing environment. The difficulty is in presenting this option as a special type of canvas - we don't want people to make this type of Youtube canvas unless they are building a Delft exhibition.

Example manifest is at https://digirati-co-uk.github.io/delft-exhibition-converter/converted/novieten.json