- add support for \ddddot
- add partial support for inverse diagonal dots (i. e. they are recognized and
displayed correctly, but the needed package is not loaded, because it would
need an off/auto/on setting as amsmath, and that calls for a general solution)
git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@30162 a592a061-630c-0410-9148-cb99ea01b6c8
