Support all lyx files (support goes to lyx-0.6 the first public available lyx version).
Fine tune the guessing of the lyx version used to produce the document (only useful for 2.10 and 2.15 documents).
Support to convertion to utf-8 encoding was added but it is not (yet) active.
git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@14558 a592a061-630c-0410-9148-cb99ea01b6c8
