More consistent exposure of elements laid out as mrow

[fred-wang]

cc @asurkov

The elements below are laid out as an <mrow> and I think it makes sense to treat them as "grouping elements". I'm not sure how other browsers expose them or how screen readers expect them to be exposed, but I wonder if we could just use the same template for all of them, that is similar to <mrow>:

ATK
Role: ATK_ROLE_SECTION
Object Attribute: tag:[their tag name]

AX API
AXRole: NSAccessibilityGroupRole
AXSubrole: AXMathRow

Here are the elements:

• annotation-xml (currently AXSubrole: TBD)
• maction (currently AXSubrole: TBD)
• merror (already exposed that way)
• mpadded (currently AXSubrole: TBD)
• mrow (already exposed that way)
• mstyle (already exposed that way)
• semantics (currently AXSubrole: TBD)
• mphantom (not exposed with default style, but see original discussion in Mapping for math and mphantom tags are missing #9)

Additionally, <mprescripts> and <none> are also laid out as <mrow> but are merely helpers to determine delimitate prescripts/postscripts and absent scripts. Will handle them in a separate issue.