Adding progress dialog doesn't appear when dragging files to data composition

1. Create a new data composition
2. Drag a large directory from a file manager
3. Observe that there is no adding progress modal
4. Observe that it is possible to crash the program by clicking the folder mid-addition and attempting to remove it

This seems like a case of looking at how the Add button code path brings up the dialog and making sure the drag code path hits it too.