- introduce option to suppress the LaTeX package mhchem, fileformat change

- introduce option to suppress the LaTeX package mhchem, fileformat change

The fileformat change is also needed because of our mistake that we load mhchem since LyX 1.6.4 automatically so that files created with LyX 1.6.4 won't compile under LyX 1.6.3. Now at least all files created or opened with LyX 1.7 can be reverted to the fileformat of LyX 1.6.x and can therefore be read by all LyX 1.6.x versions.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@31941 a592a061-630c-0410-9148-cb99ea01b6c8