Extend SwipeHelper to support vertical gestures

* Un-hardcode CONSTRAIN_SWIPE and FADE_OUT_DURING_SWIPE, define
  through proper callback, per-layout
* Up (GESTURE_NEGATIVE) and down (GESTURE_POSITIVE), mostly needed
  by new Hover hide /w swipe up gesture

Signed-off-by: Carlo Savignano <stevewatersy@gmail.com>
Change-Id: I68c42a8ccabca73a1b241ba7cf264db9c912fccd
HOVER: Implement hide only on swipe up

* Add an Y axis HideHelper, just extend current X axis SwipeHelper and
  change interested methods behaviors
* Some clean up and indenting

Signed-off-by: Carlo Savignano <stevewatersy@gmail.com>
Change-Id: I42a260ea993c614b0b9b4558a95dd44dd517c4f1