When exporting to latex with a length that does not include entirely

When exporting to latex with a length that does not include entirely
a special phrase, simply export the segment as normal text. For other
uses of these methods, end_pos is -1, so the original behavior is unaltered.
This allows for finding properly the LyX, LaTeX and other special phrases
with Advanced Find, when ignore format is off.
This complements the other changes made to latex export methods, for
exporting to latex with a given final end position.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@37599 a592a061-630c-0410-9148-cb99ea01b6c8