Track click start and end coordinates.
Add click end handler stubs.
Buggy v1 of selection square.

* First corner doesn’t consistently match clickStartX/clickStartY
position.
* “event.x” reference is undefined in FF. Should pass event via
function args instead of using global ‘event’ object.
* Square doesn’t start drawing consistently on first try.
Set canvas color for debugging.
Bind and listen for events on div#canvas, passing event via data object.
Add canvas element.