Changes

Summary

1. tuned options --- avoid confusion with "isabelle build -b";
2. tuned signature;
3. isabelle update -u path_cartouches;
4. merged
5. updated documentation;
6. more options; tuned messages;
7. tuned message;
8. isabelle update no longer uses PIDE dump, but regular session build database: more scalable; misc tuning and clarification;
9. more robust;
10. more operations; more robust;
11. proper Node.init_blobs, not just edits (amending ca872f20cf5b);
12. tuned signature;
13. tuned signature;
14. clarified session sources: theory and blobs are read from database, instead of physical file-system;
15. tuned;
16. clarified signature: more operations;
17. clarified signature: more operations;
18. tuned;
19. more direct access to session_sources, without somewhat fragile file-system operations;
20. tuned;
21. tuned signature;
22. tuned signature: avoid confusion with Document.Node.Blob and Command.Blob;
23. clarified signature: old node is ignored;
24. tuned;
25. clarified signature;