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.
Update changelog.txt

Updated changelog.

Misc documentation fixes.
Merged upstream changes, fixed conflicts
Merge pull request #392 from msalsbery/Double-Tap

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