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 on receiving focus
**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

This can currently be used in combination with the [OpenSeadragonViewerInputHook plugin](https://github.com/msalsbery/OpenSeadragonViewerInputHook) to control the viewer's default handling of user input events (feature requested in issues #167 and #246)