removeChild(child, keepalive) {
		if (this.hasChild(child)) {
			defer(deferredRemove, this, child, keepalive);
		} else {
			throw new Error("Child is not mine.");
		}
	}
Invokes the removeChildNow in a defer, to ensure the child is removed safely after the update & draw stack has completed. <br>
if the given child implements a onDeactivateEvent() method, that method will be called once the child is removed from this container.
@param {Renderable|Sprite|Collectable|Trigger|Draggable|DropTarget|NineSliceSprite|ImageLayer|ColorLayer|Light2d|UIBaseElement|UISpriteElement|UITextButton|Text|BitmapText} child - Child to be removed
@param {boolean} [keepalive=false] - true to prevent calling child.destroy()