misc tuning;

tuned Change: eliminated redundant copy of document id;

eliminated redundant session documents database; tuned;

use isabelle.Future; tuned;

renamed current_document to recent_document (might be a bit older than current_change); Change: explicit future value of Document.Change instead of implicit lookup in Session database; Document_Model: apply Document.text_edits here (as future);

renamed Proof_Document to Document;

tuned dialog;

improved dialog;