A preferred grammar is selected when a user visits the Minibar for the
first time. (Like before, Minibar remembers the selected grammar for future
visits.)
A preferred list of grammars can be specified in config.js in the .../minibar
directory on the server, e.g. like this:
preferred_grammars=["/grammars/Foods.pgf","/grammars/ResourceDemo.pgf"]
The first available grammar from the list is used.