Syntax of Form Constraints

Syntactical Ideas:

• Similar interface for specifying form-fields like in non-positional arguments
  Use e.g. some-field:boolean,required as usual
  
• Simple syntax, address non-programmers as well (not so many curley braces like ad_form)
• Multiple refinements of  form-fields possible
• Technically: Tcl list of form-field-specs