Fix bugs #6047, #4043, and #5394.

Fix bugs #6047, #4043, and #5394.
6047: Lyx 1.6.3 unable to typeset the third chemical equation of the
      file mhchem.lyx (package mhchem)
4043: mhchem support
5394: support for the mhchems's \ce command
The \ce and \cf insets are text mode environments that allow entering
spaces and mathmode commands. LyX leaves them alone and doesn't try to
be smart, i.e., the behaviour is exactly the same we had in the old days
with text-in-math mode environments, before the introduction of the
\ensuremath and \lyxmathsym macros. This means that in those environments
one has to know what he is doing, as LyX will not offer any protection.
The hack of enclosing \ce and \cf in a \text{} environment in order to
be able to enter spaces is no longer necessary.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@30338 a592a061-630c-0410-9148-cb99ea01b6c8