scrolledwindow: Handle directional cancellation if there is a single scroll direction

scrolledwindow: Handle directional cancellation if there is a single scroll direction

A pan gesture is optionally attached if there is only one scrolling direction, the pan
gesture orientation is changed so movements tangential to the scroll direction get
scrolling cancelled (The pan gesture is automatically denied when that happens, and
that state change spreads to the others gestures in the group). If the pan direction
happens in the expected directions, no cancellation happens, and scrolling eventually
takes place.