Fix bug 1826 + A more precise parser for bibtex files based on the description found...

Fix bug 1826 + A more precise parser for bibtex files based on the description found on this web page: http://artis.imag.fr/~Xavier.Decoret/resources/xdkbibtex/bibtex_summary.html

Most important improvements:
- read file character by character and treat newline like space characters.
  Thus the @ does not need to be at the beginning of a line.
- it reads @string entries and replaces the strings in the field values
- it also handles @comment entries
- it recovers from syntax errors at the next entry starting with @
- it ignores everything between entries

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17723 a592a061-630c-0410-9148-cb99ea01b6c8