This preference allows users to override the related option in Mac OS X's
Mouse/Trackpad preferences. This effectively lets the user determine
which "context" all of X11 fits into for context-based scrolling until
such API exists within X11 itself to pass along to X11 clients.
This is applicable to Mav OS X 10.7+
Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>