moveUp(child) {
		const childIndex = this.getChildIndex(child);
		if (childIndex - 1 >= 0) {
			// note : we use an inverted loop
			this.swapChildren(child, this.getChildAt(childIndex - 1));
			// mark the container as dirty
			this.isDirty = true;
		}
	}
Move the child in the group one step forward (z depth).
@param {Renderable|Entity|Sprite|Collectable|Trigger|Draggable|DropTarget|NineSliceSprite|ImageLayer|ColorLayer|Light2d|UIBaseElement|UISpriteElement|UITextButton|Text|BitmapText} child -  Child to be moved