InsetMathBox: split the boxed text into <mtext> and other tags, while boxing the...

InsetMathBox: split the boxed text into <mtext> and other tags, while boxing the whole inset within an <mrow>.

Rationales:
- previously, <mstyle> was used, but it's being deprecated for MathML 4 Core in favour of CSS and <mrow> (not a big deal in itself)
- the whole box cannot be hosted within the same tag, because neither <mstyle> nor <mrow> can have text, they need an intermediate container, <mtext> (which cannot hold anything else that pure text)
- new behaviour: always output a container for the whole box that has the right attributes, i.e. an <mrow>; split the content of the cell to have text and other tags set apart (text in <mtext>, other tags left as they were)

Old behaviour, invalid MathML (2 to 4):
<mstyle XXX>text<mn>.</mn></mstyle>

New behaviour, valid MathML:
<mrow XXX><mtext>text</mtext><mn>.</mn></mrow>