tuned signature -- avoid warning during jEdit startup;
more thorough change of syntax style extender: jEdit.propertiesChanged invalidates buffer chunk cache;
clarified startup sequence;
more robust;
more robust: fall-back for SyntaxUtilities.StyleExtender when Isabelle plugin is unloaded;
separate base plugin for important services that should be always available, despite startup errors of the main plugin; --HG-- rename : src/Tools/jEdit/src/isabelle_encoding.scala => src/Tools/jEdit/src-base/isabelle_encoding.scala