Fix bug #9554: When we save a file that was not originally in the current

Fix bug #9554: When we save a file that was not originally in the current
LyX format, create a backup of the original file. We put it in the backup
directory, if one exists, otherwise in the directory the original file is
in. This is the same strategy as for normal backups. Basically, the only
diferences are: (i) what name we use and (ii) we do not over-write any
backups that may already exist.