Make use of otexstream also in external::writeExternal

Make use of otexstream also in external::writeExternal

This was not necessary when LyX was generously outputting newlines.
As it may happen that the output produced by writeExternal (the
result of an external inset) starts right at the beginning of a
line, if otexstream does not know that something is already on
the line, the iomanip-like variable 'breakln' fails to actually
break the line.