stackable-box: Swipe to higher children only from borders
HdySwipeable.get_swipe_area() a directyion param, allowing to discriminate the swipe froward and swipe back areas, then use it in
HdyDeck to makes swiping forward with the over transition or back with the
under transition possible only from the border, while still allowing to swipe from anywhere otherwise.
Fixes #167 (closed)