Make assistive-mml unset speech/braille and vice-versa

[dpvc]

This PR adjusts the menu items for speech/Braille and hidden MathML so that selecting hidden MathML will turn off speech and Braille, and vice versa. That makes is a little easier to switch the type of assistive support to be used. (It really doesn't make sense to have both speech/braille and assistive MathML on at once in any case.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.49%. Comparing base (685a53b) to head (c084317).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1388   +/-   ##
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  Coverage    86.49%   86.49%           
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  Files          340      340           
  Lines        85813    85813           
  Branches      4815     4815           
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  Hits         74220    74220           
  Misses       11593    11593           

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[zorkow]

As we are using it already in 4 places, would it make sense to move the pretend we're loading hack into a separate method like

function withoutRerendering(exec: () => void) {
  Math.loading++;
  exec();
  Math.loading--;
}

[dpvc]

I've added the function you requested (I had meant to do something like that and forgot). I also added some text to the help dialog indicating the new menu behavior.

[zorkow]

lgtm.