Ctrl+Click+Drag Scrolling
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.
Some more modifications.
These are mostly style fixes.
Functions dealing with private data members probably don't need to be virtual.
Using const when possible.
Removing the Zoom factor call because there is a zoom factor variable that isn't used, so it's a little confusing.  Instead don't really need it.

Using 'using super = ' to reference the super class.
Merge pull request #72 from calband/ctrlDragScrolling

Ctrl+Click+Drag Scrolling (#66)