Returns the group bounding box.
Behaves like {@link Ext.draw.Sprite#getBBox} method.
@return {Object} an object with x, y, width, and height properties.
function() {
        var i = 0,
            sprite,
            bb,
            items = this.items,
            len = this.length,
            infinity = Infinity,
            minX = infinity,
            maxHeight = -infinity,
            minY = infinity,
            maxWidth = -infinity,
            maxWidthBBox, maxHeightBBox;
        
        for (; i < len; i++) {
            sprite = items[i];
            if (sprite.el && ! sprite.bboxExcluded) {
                bb = sprite.getBBox();
                minX = Math.min(minX, bb.x);
                minY = Math.min(minY, bb.y);
                maxHeight = Math.max(maxHeight, bb.height + bb.y);
                maxWidth = Math.max(maxWidth, bb.width + bb.x);
            }
        }
        
        return {
            x: minX,
            y: minY,
            height: maxHeight - minY,
            width: maxWidth - minX
        };
    }