display list depth manager , this class originally from http://www.libspark.org/browser/as3/DepthManager with modification in removeChild function
function addChild( child:DisplayObject ):DisplayObject
function addChildAt( child:DisplayObject, depth:int ):DisplayObject
function setDepth( child:DisplayObject, depth:int ):void
function getDepth( child:DisplayObject ):int
function swapChildren( child1:DisplayObject, child2:DisplayObject ):void
function removeChild( child:DisplayObject ):DisplayObject
function removeChildAt( depth:int ):DisplayObject