Fix moving `.editable` from root block to Controller #typo
Move mousedown handler back to container

4fd5fcf "Don't stopPropagation() mouse events" moved the mousedown
handler from the container to the root block just inside it, breaking
the test in `visual.html` "Clicks/mousedown to drag should work anywhere
in the blue box", where mousedown events outside the math field are
trigger'd on the math field's container.