[BUG #8504] qx.Mobile List Group Header should be sticky
* Default to position static if sticky is not supported, for keeping sure "top" does not move all elements.
[BUG #8505] qx.Mobile Scroll container should fire momentumStart/momentumEnd event
* Added detection of momentum, and firing "momentumStart"/"momentumEnd".
Refactored _getParentScrollContainer to be able to be included as a Mixin "MScrollHandling"