removed unused Session.prover_logic; slightly more robust Session.start/stop; added Session.create_id (formerly in Isabelle_System); Change/Command/Proof_Document: uniform id value, not implicity creation; eliminated Proof_Document.is_command_keyword -- plain method via explicit session;

class Session models full session, with or without prover process (cf. heaps, browser_info); replaced Prover by Session, with a singleton instance in Isabelle plugin (shared by all active buffers); misc cleanup of Session vs. Plugin instance; eliminated Prover_Setup -- maintain mapping of JEditBuffer <-> Theory_View directly; misc tuning and simplification;

tuned;