Sets a content provider on a `GtkDragSource`.
When the data is requested in the cause of a DND operation, it will be obtained from the content provider.
This function can be called before a drag is started, or in a handler for the [[email protected]:GtkDragSource:prepare]
signal.
You may consider setting the content provider back to null in a [[email protected]:
GtkDragSource:drag-end] signal handler.
| this |
a `GtkDragSource` |
| content |
a `GdkContentProvider` |