Change backspace behavior in align type equations

Change backspace behavior in align type equations

When hitting backspace in the first position of a cell in mathed,
if the cell is part of an array, the whole array is dissolved
and the sole cell content pasted in the containing environment.

But if the cell is part of an eqnarray or align math hull type, the
cursor is simply (and unexpectedly) taken out of the whole equation,
which is left unchanged.

This patch makes backspace equivalent to the cursor left key in those
cases, while leaving the behavior unchanged in all other ones.

Fixes #11678