Implement #116 - Add RegEx validation to properties

This implements support for RegEx validation for each property in a fieldset. 
The RegEx validation is implemented as a supplement to the "required" validation; thus RegEx validation is only performed if the property actually has a value. This way we can have optional properties that conform to a RexEx validation if filled out.
Localized the input field label for Danish, default English localization for the rest of the languages.