major code cleanup for fractions:

major code cleanup for fractions:

- InsetMathFrac.cpp/h:
    - InsetCFrac, InsetDFrac, and InsetTFrac are now included as kind of InsetMathFrac
    - InsetDBinom and InsetTBinum are now included as kind of InsetMathBinom
    - support for the optional argument of \cfrac (cursor position still needs to be fixed, see post in mailing list)

- stdtoolbars.inc:
    - add entries for \cfrac
    - remove entry for \atop since atop is outdated and only in LaTeX2e for compatibility reasons (according to the LaTeX companion, you get warnings from amsmath in the LaTeX log when using atop)

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@29114 a592a061-630c-0410-9148-cb99ea01b6c8