Don't .stopPropagation() 'cut' for no reason

As brought up in #53, when pretending to be a math textbox, shouldn't
.stopPropagation() any of the events emanating from the hidden textarea
at all.

Originally .stopPropagation()-ed 'cut' to fix \MathQuillMathField{}
(then called \editable{}), 8b7ea480552f8b609f829bdc90666d694d2fe621,
but that became unnecessary early-midway thru #233,
a444fde65a240738e0e40efd49fa0dfafe3dc9d0, where 'cut' and 'paste' events
were .preventDefault()-ed only ever if they came directly from a static
math textarea, rather than anywhere inside a static math root, even an
inner math field.
Don't stopPropagation() 'selectstart' in textarea

Can't just remove stopPropagation() because then the handler on the
static math, whose closured `textarea` variable would be different,
would selection of inner math field's hidden textareas. Now we instead
allow selection in any textarea, and prevent it everywhere else.