'PATH prefix' LyX preference

[Michal]

    I haven't checked it myself, but from the recent discussion with
Diego Queiroz I conclude that the name 'PATH prefix' from LyX
preferences is misleading, because it is evident that PATH isn't
prefixed by it.

    Instead of changing a name, I'd rather change the behavior of LyX to
really prepend the PATH instead of appending to it - otherwise there are
conflicts sometimes (like with the ImageMagick 'convert' conflicting
with system 'convert' utility).

Regards,
Michał