Simplification of the menclose element

[fred-wang]

See also #104 and #3

I would like to discuss how much the menclose notations are used, whether they could be replaced with CSS etc

In practice, my feeling is that I've mostly seen (rounded) box and roundedbox (which can be replaced with CSS), updiagonalstrike and circle.

Also, I don't like that some notations are not direction/writing-mode friendly.

I think menclose is the kind of candidate for "custom MathML element" where people could extend mrow with their own drawing of special enclosing/striking notations.

[fred-wang]

The fact that "menclose" is present in MathML Core but that there is no plan to implement it in Chromium has been a source of confusion when discussing with externals (TAG, Google, a11y people...) or when writing WPT tests. I've also never been satisfied with the current description which is not as good as the rest of the spec. So moved the current draft into

w3c/mathml-core#24

in order to avoid polluting the current text with low-quality WIP things and many "ISSUE" labels. Let's go back to this when the future of menclose is clearer.