Add document level validation

A client suggested an idea that supports inter-field validation. E.g. to check that a start date is not past an end date. For this we could implement a form of document level validation.

A rough first idea is: on save (both in the wicket as well as the angular editor) the backend executes some code that validates the jcr data that is saved in the draft variant. That can be a validator on document node level (specific validators per document type), but there can also be validators on compound level (startdate/enddate compound used in multiple document types).