explicit Change.Snapshot and Document.Node; misc tuning and clarification; Document_View: explicitly highlight outdated command status;

simplified/refined document model: collection of named nodes, without proper dependencies yet; moved basic type definitions for ids and edits from Isar_Document to Document; removed begin_document/end_document for now -- nodes emerge via edits; edits refer to named nodes explicitly;

somewhat uniform Thy_Header.split_thy_path in ML and Scala;

uniform naming of imports (source specification) vs. parents (thy node names) vs. parent_thys (theory values);

load_thy/after_load: explicit check of parent theories, which might have failed to join proofs -- avoid uninformative crash via Graph.UNDEF;

export use_thys_wrt;