Allow (embellished) operators in \underset, \overset, and \underoverset to specify accent="true"

[dpvc]

Currently, \underset, \overset, and \underoverset force accent="false" on mo elements that are set on top or bottom of the base. This PR allows a (possibly embellished) operator that has an explicit accent="true" to set the accent or accentunder attributes of the munder, mover, or munderover that is produced. That allows, for example

\underset{\mmlToken{mo}[accent="true"]{⎵}}{abcabc}

to properly handle the underbrace as an accent (i.e., without reducing its scale). (Without this PR, the accent always gets accent="false", and so is reduced in size. There is an additional bug where that causes the stretchy character not to stretch sufficiently, but that is treated in the fix-stretchy-widths branch.)

[zorkow]

lgtm.