better mapping of cocoa concepts to vtk concepts

This issue was created automatically from an original Mantis Issue. Further discussion may take place here.

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Assuming VTK uses the same concept as X11 and Win32 in their api (which is very likely), I suggest mapping these concepts for Cocoa...

NSView -- VTK WindowId n/a -- VTK DisplayId NSWindow -- new VTK concept, or keep it in Cocoa/Carbon implementations. Its possible for the code to get an NSWindow from an NSView, so one shouldn't be required to provide that info to vtkCocoaRenderWindow.