edit_document: synchronous reply to ensure consistent state wrt. calling (AWT) thread;

distinguish proper Isabelle_Process INPUT vs. raw STDIN, tuned corresponding method names; asynchronous Isabelle_Process.init -- raw ML toplevel stays active; simplified Isabelle_Process using actors; misc tuning;

added Library.thread_actor -- thread as actor;

clarified JEDIT_JAVA_OPTIONS vs. JEDIT_SYSTEM_OPTIONS -- discontinued JEDIT_APPLE_PROPERTIES; static default Dactors.corePoolSize=4 for more reproducible concurrency, independently of number of cores;