Fix TextBlock's rendered from LaTeX useless on master

Thanks for pointing this out in #138, @zb3!

@jayferd whoops, this something that, before the `refactor.environments -> dev -> master` musical chairs, was broken on `dev` but fixed in `refactor.environments`, in 780c5291017cbfd4a31fbd4797b13fa36419010c, that really should've been fixed before we made `dev` into `master`.

Here's what's annoying--the fix involves lots of things that merge-conflict with `refactor.directions`, as I discovered attempting to rebase 780c5291017cbfd4a31fbd4797b13fa36419010c onto `master`. It's not just this rebase that's annoying&mdash;when eventually merging in `dev` into `master`, I think that all of those will either be merge conflicts again or, worse, semantic conflicts that git doesn't even know about and instead silently are just merged wrong and continue to say `.firstChild` etc.

This branch, `fix.text-from-latex`, is instead 780c5291017cbfd4a31fbd4797b13fa36419010c rebased onto `refactor.directions` with almost all irrelevant `refactor.environments` commits removed (with one exception to save another merge conflict), and should result in very few merge conflicts while still fixing `TextBlock`s rendered from LaTeX (e.g. pasted). Unfortunately it includes the current state of `refactor.directions` which I'm not in love with, but we'll fix it on `dev` later and fixing this bug is more important.

<del>Alternatively we could fix things by just updating `master` to the current `dev`, which is believed to be stable aside from not being able to select in `TextBlock`s (but everything else works which is a huge improvement over `master` and what this is trying to fix), right?</del> Correction: on `master`, if you type in `\text`, everything works including selection, and with this `fix.text-from-latex`, if the `TextBlock` is rendered from LaTeX too (e.g. pasted), everything works too.