Support copying plain text and TX code for Math

[bhavyashah]

Wherever MathCAT supports copying (currently only when in navigation), it should offer the following copy modes:

• MathML Code (current)
• TeX Code (like in MathJax menu)
• Plain text (like when you use Speech History add-on to copy last utterance or what you view in NVDA's Speech Viewer)
  The MathCAT settings dialog should have a combo box offering these three copy modes from which the user can pick their preference. I'd probably set it to plain text sometimes and TeX code other times.
  A fourth wishlist option would be "Ask me every time" which would present all three afore mentioned copy modes in a context menu whenever the user tried to copy Math. This would eliminate the need to keep reconfiguring the copy mode from the settings dialog.

[NSoiffer]

For the TeX code, I need to write a chunk of code that probably is 1-2 week's work. I think supporting ASCIIMath also makes sense. That too is a similar amount of work. So altogether, this looks to be a significant project, so it won't happen soon. Unfortunately all the rust packages seem to be LaTeX->MathML, not the other way around.

I may get the plain text done first (separately).