Changes

Improve syntax highlighting of grammar listings

Changes include:
- Make sure only actual Modelica keywords are highlighted as such.
- Highlight fixed strings in same way as Modelica keywords.
- Make BNF syntactical constructs stand out by having upright shape (using italics for production rule names).

That production rule names are set in italics will also make it easier to tell the difference between a grammar listing and a Modelica listing when reading the document.

Highlight two classes of Modelica keywords: medium weight and boldface