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.
Separated event-model/device specific event handling code from
event-model/device agnostic event handling code.
Use enter/leave events instead of over/out events.
Updated unit tests for above changes.
Added unit tests for mouse gestures.
Updated changelog for breaking changes and deprecations.
