Ctrl+Click+Drag Scrolling (#66)

Stunt would prefer to scroll the field canvas by clicking and dragging the
mouse while holding down control (instead of just holding down ctrl) -
this commit makes that happen, but there's something for us to keep note
of for the future: if you click to drag the canvas, but you click on a
dot, it will select that dot. That is, when you drag, the current tool
that you are using is not disabled. This shouldn't be a big deal, but I
have plans to fix this up in the future.