Add Double-Click/Tap Gesture to MouseTracker (#300)

Fix for enhancement #300

Added double-click/tap gesture detection to MouseTracker.with corresponding dblClickHandler event callback.

Added unit test for double-click detection.

Added Viewer dblClickHandler handling to (optionally) zoom on double-click.

**Notes:**
The native DOM dblclick event was NOT used in this implementation due to differences in implementation between browsers.

The event sequence used by most browsers was used: *down-up-click-down-up-click-dblclick*. As with the native DOM dblclick event, the click and dblclick events should not both be subscribed to. It can be done, but a delay must be introduced to wait and see if a click is going to become a double click.