Fixed: refactored key scrolling with switch

This is cleaner and has slightly better performance.
Fixed: plugin click handlers have access to diva instance

We now pass a reference to the Diva instance when executing plugin handleClick functions, so that it is possible to execute public Diva methods when a plugin is activated.
Fixed: canvas mode keyboard scrolling works as expected

Previously, scrolling via the keyboard in the canvas plugin continued to scroll Diva in the background. Now, the arrow keys scroll the image on the canvas. Pgup/pgdown are disabled as per discussion in the issue.

Fixes #207