#3 MathML compliance

[Jérôme Joslet]

Add more of the MathML compliance.
It includes:
* mstyle attributes and other presentation element attributes attached to mstyle element.
* support more stretchy operator: arrows, etc.
* movable limits for operators (sum, prod, etc.)
* linebreaking
* mglyph, mmultiscripts and mpadded elements
* bevelled mfrac
* all mtable attributes
* maction
* Style: background, mathstyle, style for all elements