Add "save action" "convert to bibtex"

[koppor]

The save actions currently offer "field formatters" only. We should add bibtex conversion. Use case: A research group manages its bibliography using biblatex format. When publishing, publishers demand bibtex. Thus, the .bib file should be of bibtex format.

• Refs Support of different type of BibTeX files jabref#160 (because this are different types of .bib files)