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.
All event handler signatures changed to 'fn(event)'
eventSource property added to the event object passed
Redundant event source properties removed ('button' and 'viewer'
Documentation updated in MouseTracker
changelog.txt updated