evdev: Lookup config to report touchpads as such

Check a touchpad-only setting, and if it returns an expected value there,
the device must be a CLUTTER_DEVICE_TOUCHPAD.

https://bugzilla.gnome.org/show_bug.cgi?id=741350
x11: Resort to device name matching for non-mt touchpads

If a touchpad is not multitouch, or does not report MT axes (eg. through
the libinput driver), resort to name matching before falling back to
CLUTTER_POINTER_DEVICE.

https://bugzilla.gnome.org/show_bug.cgi?id=741350