* prefs2prefs.py : Use a list of [format, conversions] pair.

* prefs2prefs.py : Use a list of [format, conversions] pair.

    Use a list of [int, list of functions] pair, representing
      - int: the format number, and
      - list of functions: what needs to be done to the file to
                           update it to the given format number.

    This matches what is done in lyx2lyx and helps keeping track of
    format numbers and their corresponding conversion routines.

    Also, add another sanity check.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@39668 a592a061-630c-0410-9148-cb99ea01b6c8