support 'new' constructor notation

The SDF3 to SDF2 conversion fails on productions with ‘new’ constructors, i.e. without the cons indicator:

    context-free syntax
      Int -> PreTerm {"Int"}

Cause: the SDF2 pretty-printer that is used does not recognize the Constructor constructor. Fixed

Also: color such constructors blue, just as is done in the SDF2 editor.
Merge branch 'unstable'