Refine the zoom behavior of the Navigate Tool

[Keavon]

Extends upon the work on #155.

It would be great to slightly refine this behavior and make it so zooming is relative to the position where you clicked down, and also a click without dragging should zoom in to the next increment (IncreaseCanvasZoom) while holding Shift and clicking does DecreaseCanvasZoom. Except unlike IncreaseCanvasZoom/DecreaseCanvasZoom it zooms relative to the place you click.