Added preventDefaultAction flag to MouseTracker event objects

Added a boolean *preventDefaultAction* property (default false) to the
event object passed to MouseTracker handler methods. Implemented in the
following MouseTracker subscribers:

* Viewer.keyboardCommandArea.innerTracker.focusHandler:
preventDefaultAction == true prevents scrolling viewer into view
* Viewer.keyboardCommandArea.innerTracker.keyHandler:
preventDefaultAction == true prevents viewer keyboard navigation
* Viewer.innerTracker.clickHandler: preventDefaultAction == true
prevents viewer zoom on click
* Viewer.innerTracker.dragHandler: preventDefaultAction == true prevents
viewer panning with mouse/touch
* Viewer.innerTracker.scrollHandler: preventDefaultAction == true
prevents viewer zooming on mousewheel/pinch