* Make renderer separate class, so that you can actually have a renderer for
  html, PDF, XML, etc.
* Add wf capabilities.
* Describe use case to simply create a standalone form
* Move type checking to model
* find generic solution for type checking
* We should really consider making the datatype part of the field itself:
    there can be only one!
* Maybe fieldproperties settings like constraint should be a tuple, not 
  only holding the expression, but also an error message and message id