Mouse button emulation for the one-button mouse Apple machines isn't
restricted to older ADB based machines. There are PPC Powerbooks where
the keyboard and the mouse are no longer on the ADB bus but regular USB,
and users still like (and need) to be able to emulate the middle mouse
button with F11 and the right mouse button with F12.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>