Reform css 2

On top of #7308 : 

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    unify submenu style with menu style

    closes #7303.

    Also change the arrow that indicate submenu for a fontawesoem icon,
    instead of the ugly hack that show only the left border of a rectangle element
    which with is twice the one of its border to get a triangle.
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![capture d ecran 2014-12-25 a 12 12 33](https://cloud.githubusercontent.com/assets/335567/5553792/5fe7e66c-8c2f-11e4-9b6b-5dfe676924bf.png)


See last commit: https://github.com/Carreau/ipython/commit/1d38586eaa599d19b84ec361e08d1737637d8840
that drop hardcoded style (let things inherit), use variable for submenu arrow color.