Add menclose to MathML core

[fred-wang]

Opening a new issue to track needed work for menclose ( PR of the original text: #24 ).

Known issues:

• Simplify: Simplification of the menclose element mathml#105

• menclose does not specify in which order the notations are painted ( menclose does not specify in which order the notations are painted mathml#113 )

• need OpenType MATH values if a dedicated algorithm is provided ( OpenType MATH values for menclose notations  mathml#97 )

Instead of a dedicated algo, it would be better if menclose would just use the current mrow algo and rely on pure CSS for styling notations.

[NSoiffer]

I don't think order matters for the current set of values. However if it does need to be specified because of some CSS properties, the order in which they are listed seems reasonable to me.

[NSoiffer]

This was discussed at the CG meeting today. From the notes:

• FF/Webkit support menclose and almost all of the attrs
• @bkardell suggested that maybe a UA stylesheet would be a trivial way to implement some of them for level 1, but eventually withdrew his suggestion

The CG felt menclose was important and since the other browsers implement it but Igalia currently doesn't plan to do it, this should move to level 2. There should be a polyfill to handle this.