Fix tex2lyx script output

Fix tex2lyx script output

tex2lyx did not use the InsetLayout for the script insets, so it did not know
that it had to use Plain Layout. However, there is still a possible pitfall:
InsetScript uses a hard coded plain layout for the InsetText constructor, so
if anybody would remove ForcePlain 1 and MultiPar false from the InsetLayout
then LyX would still write script insets with plain layout, but tex2lyx would
output standard layout again.