Ensure good integration of MathML with the accessibility tree

[littledan]

@joanmarie has noticed some issues here with respect to the current MathML/accessibility integration. I'll defer to her for details, but it will take time to gather the relevant information. Filing this issue so people remember to come back to it.

[fred-wang]

Asking to Joanie, this can be closed and handled somewhere in other issues. We have #64 for example, and other semantics discussion happening in the MathML CG and other groups.