Changes

Summary

1. tuned headers;
2. merged
3. refer to known_theory; support for qualified theory name;
4. tuned;
5. provide session base for "isabelle build" and "isabelle console" ML process;
6. tuned comment;
7. more explicit lookup of loaded_theories: base names allowed here; no base names for known_theories;
8. support for all_known_theories of all sessions;
9. tuned;
10. known_theories from imported sessions;
11. tuned;
12. more checks;
13. tuned;
14. tuned signature;
15. tuned;
16. support for static session imports, without affect build hierarchy;
17. explicit Sessions.Selection;