Ignore drags on Qt4 TreeEditor if no associated Python object

Prior to this, dragging an object into the tree editor which could not be converted to a PyMimeData with an associated local instance caused the enter to be accepted, but all other drag events to be refused.  This effectively swallowed the events, preventing any other widget from getting them.

This change applies the same check for the existence of PyMimeData instance() in the dragEnterEvent as are used in the dragMoveEvent and dropEvent handlers.