Changes

Summary

1. clarified check, notably for bibtex theory (amending 5f082b1fa9fa);
2. NEWS;
3. proper check for imports, not just ROOT entries;
4. tuned signature;
5. implicit thy_load context for bibtex files (VSCode);
6. bibtex theory is always required and thus eventually consolidated;
7. implicit thy_load context for bibtex files;
8. clarified signature: prefer Document.Node.Name.is_bibtex;
9. avoid spurious noise: Prover IDE is meant to check for errors;
10. clarified signature;
11. avoid clash with special files in HTML output;
12. unused;
13. tuned signature;
14. prefer existing Resources.check_path;
15. more accurate bibtex entries;
16. added command 'bibtex_file' (for PIDE interaction only);
17. clarified default position for empty message pos;