ConstReadonlyConnector: "connector"A connector: a line that may be glued to other shapes' connection sites.
ReadonlyShape: "shape"A shape drawn from a preset or custom geometry, with or without text.
Every kind is an IShape whose content type is 'shape';
this is the one place the distinction survives a round trip. A JSON that states none is a
shape.
What kind of drawing a shape's JSON describes.