We'll need to standardise on the Touchpad related keys in udev, kernel, and
X.org. I selected F21 for XF86TouchpadToggle, F22 for XF86TouchpadOn and F23
for XF86TouchpadOff.
See:
https://bugs.freedesktop.org/show_bug.cgi?id=31333
Signed-off-by: Martin Pitt <martin.pitt@ubuntu.com>