SystemUI: Extend SwipeHelper

I was not happy with long press on notifications and recents
to open apps in floating mode. The long press duration kinda
breaks the UI interaction flow, which prevented me from
actually using floating windows. Therefore, I've created a
private modification, which enables floating mode for
notifications and recents by swiping to the left. This
greatly improves usability without strongly deviating from
stock UI experience, in strict accordance with PA's
philosophy.  Changes are surgically implemented to keep the
probability of merge conflicts rather low. The code was
successfully tested on mako, hammerhead, tilapia, and manta.
Let the team discuss and decide whether or not this is a
worthy extension to PA.

Changes are split into two commits (PT1 & PT2).

PT1:
- Extending SwipeHelper:
SwipeHelper can now distinguish between directions LEFT,
RIGHT, UP, DOWN and can fire an abstract trigger 'event'. A
trigger direction can be specified and when a swipe in this
direction occurs dismissable children will not be dismissed,
instead they will be snapped back.  Whenever a child is
swiped far or fast enough in that direction an event is
triggered and the onChildTriggerd() callback is issued. This
callback was added to the SwipeHelper.Callback interface and
therefore requires all its implementations to realize this
method, such as probably Hover, which is sadly not open
source (yet?).

PS7: Added Hover support

Change-Id: Ida06195913f022463d29ca22092e81fb8993ad50