[Xwayland] Add PtrFeedback to the touch device
Trying to change the acceleration/threshold on Xwayland cannot work, and
the corresponding handler xwl_pointer_control() is a no-op.
Yet, an X11 client trying to change those on the touch device may possibly cause a crash because the touch device in Xwayland doesn't set that.
Initialize the touch device's PtrFeedback to make sure that just cannot
happen.