Fix bug #6894. We save counter information during updateBuffer(), and

Fix bug #6894. We save counter information during updateBuffer(), and
then output \setcounter macros during snippet generation, so that we get
the right equation values.

Note: It would be possible to use this same machinery to fix bugs in
instant preview, e.g., that you always get things like (0.3) as equation
numbers, if you use equations numbered by section. I'll perhaps post a
patch for that.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@37186 a592a061-630c-0410-9148-cb99ea01b6c8