* Extending MediaElementEvent with a PARENT_CHANGE type, and 'parent' property.
* Having CompositeElement fire a PARENT_CHANGE event when adding or removing children.
* Having MediaElement listen for PARENT_CHANGE events, storing a private reference, throwing an exception ("Error: Element is already a child of another composite element") when the current parent is not null.
* Adding error message to OSMFStrings.
