Release todo: added userpref for Mac users having "Natural Scroll" set.

As per discussion and analysis of all trackpad usage, we now
follow this convention:

- Blender follows system setting for trackpad direction preference.

- If you set your system to "natural" scroll, we need to invert a couple
  of cases in Blender we do "natural" already. Like:

   - view rotate (the inversed option just never feels ok)
   - scroll active items in list or pulldown menu (up/down is absolute)
   - ALT+scroll values in buttons (up/down is absolute)

The new User Preference setting "Trackpad Natural" handles this.

For 2.66 we only have trackpad handling for OS X... so this isn't
affecting trackpad usage in Windows and Linux, which stick to be mapped
to Scroll Wheel still.

(Note: viewrotate now is "natural" always, changing how it worked in the
past weeks).

release/scripts/startup/bl_ui/space_userpref.py

@@ -963,6 +963,8 @@ class USERPREF_PT_input(Panel, InputKeyMapPanel):
         return (userpref.active_section == 'INPUT')
 
     def draw_input_prefs(self, inputs, layout):
+        import sys
+
         # General settings
         row = layout.row()
         col = row.column()
@@ -1015,6 +1017,11 @@ class USERPREF_PT_input(Panel, InputKeyMapPanel):
         sub.prop(inputs, "invert_zoom_wheel", text="Invert Wheel Zoom Direction")
         #sub.prop(view, "wheel_scroll_lines", text="Scroll Lines")
 
+        if sys.platform == "darwin":
+            sub = col.column()
+            sub.label(text="Trackpad:")
+            sub.prop(inputs, "use_trackpad_natural")
+
         col.separator()
         sub = col.column()
         sub.label(text="NDOF Device:")