BUG: #2788. Crosshairs change position on zoom (and pan)

Added OnSliceNodeModifiedEvent() to update the Crosshair display
position when SliceNode is modified, for instance by zooming or
panning.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22200 3bd1e089-480b-0410-8dfb-8563597acbee