Better TabularEditor Drag and Drop in Qt

Currently the drag methods for a TabularEditor in Qt just generate a list of indices being moved (as a string, no less).  This means that the TabularAdapter methods such as get_drag() are not supported, and objects cannot be dragged out of a TabularEditor under Qt.

This PR changes the mime data generated by the TabularModel to a PyMimeData with a list of the actual objects generated by get_drag() for the selected rows, and adds the original list of indices to this so that the internal drag and drop still works.