The device_added_cb is called once for each tool added. The wacom driver
hotplugs tools in the order stylus, eraser, cursor, pad.
update_current_page will add a new page once a tablet has stylus and
eraser, before cursor and pad exist. priv->pad is thus always NULL,
causing, cc_wacom_page's update_tablet_ui to remove the "Map Buttons..."
button for any device.
Change the code to update the tool list for every new tool we get,
merely triggering the visibility of the button instead of destroying it
completely.
https://bugzilla.gnome.org/show_bug.cgi?id=672691
