Asks the TreeDragSource to delete the row at
path, because it was moved somewhere else via drag-and-drop. Returns false if the deletion fails because
path no longer exists, or for some model-specific reason. Should robustly handle a path no longer found in
the model!
Asks the TreeDragSource to fill in
selection_data with a representation of the row at path. selection_data->target gives the
required type of the data. Should robustly handle a path no longer found in the model!
Asks the TreeDragSource whether a particular
row can be used as the source of a DND operation. If the source doesn't implement this interface, the row is assumed draggable.