The ongoing drop operation over the controller's widget or its descendant.
If no drop operation is going on, this property returns null.
The event controller should not modify the @drop, but it might want to query its properties.
When handling crossing events, this property is updated before gtk.drop_controller_motion.DropControllerMotion.enter, but after gtk.drop_controller_motion.DropControllerMotion.leave is emitted.
Get drop property.