LyXFunc::dispatch(): don't use a buffer pointer set at the beginning of the method...

LyXFunc::dispatch(): don't use a buffer pointer set at the beginning of the method as this one could change depending of the LFUN. Instead get the proper Buffer in all cases.
The side effect of this patch is that the undo/redo action are properly recorded only for document Buffers. But as this was a mess already with embedded work area. We might try to properly implement undo/redo for embedded work area later.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@31471 a592a061-630c-0410-9148-cb99ea01b6c8