Refactored all interactions to use pointer events instead of mouse and
touch events. Implemented pinch zooming for multi-touch enabled devices
and re-implemented pan support for pointer events.

This is mostly ready to go, but there is some odd behavior when pinch
zooming occasionally when your fingers get close to the edge of the
canvas (it makes the zooming stutter a little bit).

The EventListener class has been refactored to allow defining an element
per event. It also allows defining options on initialization or when
adding events.

Events specific to Puzzle were moved out of index and into the Puzzle
class. Other miscellaneous interactions were better organized into other
files.

Modifier selection now provides haptic feedback on touch devices. The
concept of toggling a modifier has been introduced, which can allow for
the toggling of modifier state based on interaction. For example, middle
clicking on a modifier with a mouse will toggle it as will triggering a
pointer down on a modifier followed by a pointer leave. For now, this
behavior is only used to toggle the rotation direction of the rotate
modifier, but it could be used for other things in the future.

The header/footer dropshadows now correctly always overlay the canvas
area.