Fix WindowLevelEvent handling bug in vtkInteractorStyleImage.

Currently, the style will correctly pass on Start, Reset and End
events if there is an observer and HandleObservers is true.  The
WindowLevel event will never be passed on to an observer because
the style searches for, finds, and assigns the CurrentImageProperty
ivar during the StartWindowLevelEvent.

Change-Id: I561346c48889cf54f903af54e7d4f4f80e8484c2
Merge topic 'release-fix-interactorstyleimage-window-level' into master

f4875501 Fix WindowLevelEvent handling bug in vtkInteractorStyleImage.