Fix cursor position after deleting a SupSub block

In a 2-block SupSub (i.e. with a superscript stacked on top of a
subscript):

Before, backspacing out of a superscript (or deleting forward out of a
subscript) would put the cursor at the right end of the remaining
subscript (or at the left end of the remaining superscript), which is
unfortunate because it's not symmetrical with typing back the deleted
superscript (or subscript).

Now, backspacing out of a superscript (or deleting forward out of a
subscript) puts the cursor to the left of the now subscript-only SupSub
(or to the right of the now superscript-only SupSub), from where
subscripts and superscripts can be typed back.
Deleting out of SupSub block unwraps one at at time

Rather than selecting the contents of the SupSub block being deleted out
of, which happens for no blocks anywhere else in MathQuill, deleting out
of a block of a 2-block SupSub will unwrap just that one block and turn
it into a 1-block SupSub (deleting out of a 1-block SupSub unwraps it,
as always).
Merge branch 'dev' into fix.SupSub
Merge branch 'fix.SupSub' into dev