* for a valid target to drag based on the mouse down. Override this method
* to provide your own lookup logic (e.g. finding a child by class name). Make sure your returned
* object has a "ddel" attribute (with an HTML Element) for other functions to work.
* for a valid target to drag based on the mouse down. Override this method
* to provide your own lookup logic (e.g. finding a child by class name). Make sure your returned
* object has a "ddel" attribute (with an HTML Element) for other functions to work.