clarified message;

keep PIDE.plugin for the sake of still open dockables etc. -- jEdit exits these *after* the stop operation;

keep style extender for the sake of potentially remaining token markers;

dynamic session_options for tuning parameters and initial prover options;

strict initialization of plugin.session: no user errors to be expected before session.start;

resources are part of early/strict initialization, but session_base is permissive to avoid crash of "isabelle jedit -l BAD"; PIDE._plugin indicates intialization state of Plugin; tuned;