RootMathBlock::text => Controller::exportText

Note again that centralizing as a service on the Controller eliminates
unfortunate monkey-patching from \MathQuillMathField.
RootMathBlock, begone!

RootMathCommand::createBlocks is so much simpler!

The README is all kindsa out of date, mentioning `Node::keystroke()`
barely helps.
Move Root{MathCommand,TextBlock} {rootelements => text}.js

Put of dealing with RootTextBlock for now #procrastination

It belongs there, right? The header of text.js specifically says
"classes of text blocks".
textarea.js => more specific saneKeyboardEvents.js

buncha textarea events stuff is in rootelements.js instead of
textarea.js, like focus/blur, copy, selectstart

Also rename matching unit tests.
s/manageTextarea/saneKeyboardEvents/g

and s/textareaManager/keyboardEventsShim/g, s/manager/shim/g

Recommend --word-diff-regex='.|\w+' to view diff.
Move textarea stuff to its own textarea.js

...out from rootelements.js
Controller now owns MathQuill instances' textareas

Recommend -w to view diff.
rootelements.js is dead! Long live controller.js!

All that is left is the skeleton Controller that wasn't even there
before `refactor.controller` started.

No, rootelements.js, you do not have legal standing for a wrongful life
suit. You're not even an inanimate object, you're a virtual one!
Update header comment of controller.js #comments