Initiates a drag on the source side. The function only needs to be used
when the application is starting drags itself, and is not needed when
drag_source_set() is used.
Table 3. Method parameters
The targets (data formats) in which the source can provide the data.
A bitmask of the allowed drag actions for this drag.
The button the user clicked to start the drag.
The event that triggered the start of the drag.