RichText's internal format is in fact a variant of DocBook format, requiring version attribute on the document's root element. The presence of this attribute is validated, but not the contents. We should validate the contents of it and additionally we should dispatch conversion on the value of this attribute, in addition to the document namespace used at the moment.

Motive: clear situation when the format is expanded/changed, as the field type will then refuse processing of the outdated (not upgraded!) version of the format.