Deprecate/Remove the mstyle element

[fred-wang]

I think we already agreed that we should keep mstyle in core as it is widely used. However, it was proposed to deprecate it in favor of mrow: #1 (comment)

To do that, we would first need to allow all mstyle attributes on mrow ; see #88

[fred-wang]

From https://lists.w3.org/Archives/Public/public-mathml4/2019Apr/0022.html ; "Neil: suggests removing mstyle from core."

I don't remember what was the resolution from previous meeting, but as I already said from the implementer side it's easy to just make mstyle use the same C++ classes as mrow for layout purpose and to map DOM attributes to CSS properties. So it's not really useful to remove it (for browser implementers).

From #55 it seems mstyle is still used a lot so I would keep in in Core. As Dani said in #1 (comment) we can deprecate it in Full and suggest mrow instead. #88 would be needed first (or at least allow mstyle attributes on mrow).

[NSoiffer]

Doesn't appear that anything remains to be done. Treated as mrow.