Improving recognizing encoding of a BibTeX file to load

Drawing from commits in the 'master' branch in order to improve
recognizing BibTeX/BibLaTeX files' encoding.
- 8e473758: Guessing file's encoding based on bit patterns
- fba235cf: Refactoring FileImporterBibTeX

BUG: 426856
FIXED-IN: 0.10

One effect is that opened files ASCII-only files are not directly
classified as UTF-8, but stay ASCII-encoded as far as possible by
classifying them as 'LaTeX'-encoded.