Spec should specify what char to use for accents/lines

TeX has a number of commands that correspond to mover/munder accents in MathML. The spec does not say what character to use for those accents. In some cases there are ASCII chars that could be used but also non-ASCII ones that are similar. Many of these characters should be stretchy when used with mover/munder.

At a minimum, the spec should say which (or all) of the following should be used for (stretchable) accents (some options listed) so that renderers and generators of MathML agree on what character(s) to use:

• \hat -- '^', U+0302, U+02C6
• \check -- 'v', U+0306, U+02D8
• \tilde -- '~', U+0303, U+223C, U+02DC
• \acute -- U+0027, U+00B4, U+02CA, U+0301, U+02B9, U+2032
• \grave -- U+0060, U+02BC, U+02CB, U+0300
• \dot -- '.', U+00B7, U+02D9, U+0307, and potentially others like U+2E33
• \ddot -- '..', U+00A8, U+0308
• \breve -- U+02D8, U+0306
• \bar -- '_', '-', U+00AF, U+02C9, U+0304, U+0305, U+0332, U+FF3F
• vec -- U+20D7, U+2192, U+27F6

Note: based on experience with MathPlayer, many of these alternatives were encountered "in the wild" so it is important that Core specifies these (MathML 3 should have) as people are having to guess what character to use.

• \overline -- should be same as \bar
• \underline -- same as \bar?