Old issue with OSX Cocoa code: shift+scrollwheel should send
a 'horizontal wheel' event to Blender. Blender only recognizes
scroll events in general though. The old code then just didn't
send an event at all, not passing on shift+scrolls.
Now the scroll event is sent anyway, relying on Blender's
keymapping to define what to do with shift+scroll.
This fixes things like shift+scroll to scale ListBox widgets.