Minibar: make it possible to configure a list of preferred grammars

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.

src/www/minibar/minibar_online.js

@@ -21,6 +21,9 @@ var minibar_options= {
     try_google: true
 }
 
+if(window.preferred_grammars)
+    minibar_options.preferred_grammars=preferred_grammars
+
 if(/^\?\/(tmp|grammars)\//.test(location.search)) {
     var args=decodeURIComponent(location.search.substr(1)).split(" ")
     if(args[0]) online_options.grammars_url=args[0];