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