Support touchpad configuration through device properties.

This patch is based on Ubuntu's touchpad configuration.

It adds a new tab to gnome-mouse-properties that allows to enable/disable the
touchpad, tapping and select between edge scrolling and two-finger scrolling.

It adds the following gconf keys:
    /desktop/gnome/peripherals/touchpad/disable_while_typing (boolean)
    /desktop/gnome/peripherals/touchpad/tap_to_click         (boolean)
    /desktop/gnome/peripherals/touchpad/horiz_scroll_enabled (boolean)
    /desktop/gnome/peripherals/touchpad/scroll_method        (integer)
    [0,1,2 for disabled, edge and two-finger scrolling, respectively]

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>