Support Grouping Doctypes with Properties

We need to support template authors to make it easier to reference information in related doctypes. Currently there is

• Doctype
• Docsubtype
• is-a

To define the type of a document instance. Since is-a demands that the referenced doctype really exists, we need to introduce

• is-of

to define a virtual family relation. Furthermore we add

• in-context

to define a virtual context within which the doctype is defined. This is typically related to the bundle the doctype is part of. We will use the shortId of the add-on as default value. With the context the intended collection of doctypes can be easily selected (i.e. in case the doctype is to any reason not unique - although there will probably more problems to this to discover).

The use case for is-of would be to grab from any doctype that belongs to a given virtual family. This is pretty much the same as is-a with the exception that the value may be an arbitrary string.