Prevent invalid latex for multilingual sections and inputenc == auto.

Prevent invalid latex for multilingual sections and inputenc == auto.
We output the \inputencoding command before the section if possible, and we
ignore all encoding changes in the section. Commands from the unicodesymbols
file will be used for characters that can't be encoded in the current
encoding instead of switching the encoding.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17827 a592a061-630c-0410-9148-cb99ea01b6c8