ML support for generic graph display, with browser and graphview backends (via print modes);
reverse graph layout (again), according to the graph orientation provided by ML;
simplified scala types -- eliminated unused type parameters;
more explicit Model.Info, Model.Graph;
renamed isabelle.graphview.Graphview_Frame to isabelle.graphview.Frame in accordance to file name;
removed obsolete Graph_XML and Tooltips;
tuned graphview command line;
more generous JVM resources via GRAPHVIEW_JAVA_OPTIONS;