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!
| this | |
| path |
row that was dragged |
| selection_data |
a SelectionData to fill with data from the dragged row |
|
true if data of the required type was provided |