Changes

Changes from Mercurial (hg http://isabelle.in.tum.de/repos/isabelle/ default)

Summary

1. merged
2. more errors;
3. clarified inversion of file name to theory name, notably for Windows;
4. clarified import_name: observe directory notation more strictly;
5. tuned signature;
6. clarified signature -- removed pointless operations;
7. clarified signature -- removed unused content;
8. clarified theory imports completion, based on session directories and current master directory (no support for local session-subdirectories);
9. find theories via session directories only -- ignore known_theories;
10. tuned;
11. tuned message;