Style classes are simple strings that can be attributes to a datatype, and which will e.g. appear on the 'class' attribute for an HTML input widget.
Generic property, calls set-method with given name on DataType.
Currently, the pattern is interpreted as a Condition for Strings and for DateTimes it
is interpreted a format indication (like the pattern of SimpleDateFormat). In this
last case there can also be more than one pattern: for every locale a different one.