Fix transfer annotation for tree_model parameter of gtk_tree_get_row_drag_data() and add nullable, optional
Fix transfer annotation for tree_model parameter of gtk_tree_get_row_drag_data() and add nullable, optional

The GtkSelectionData does not own the model set with
gtk_tree_set_row_drag_data so change the out param to "transfer none".
This fixes a crash after multiple DnD actions due to the bindings
stealing a reference each time this function is called.

This also adds nullable and optional annotations for tree_model and path while at it.

https://bugzilla.gnome.org/show_bug.cgi?id=743193

davidgumberg/gtk,Lyude/gtk-,davidgumberg/gtk,Lyude/gtk-,jigpu/gtk,alexlarsson/gtk,davidgumberg/gtk,Adamovskiy/gtk,jigpu/gtk,jadahl/gtk,grubersjoe/adwaita,jadahl/gtk,Lyude/gtk-,grubersjoe/adwaita,jigpu/gtk,alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita,chergert/gtk,chergert/gtk,jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,davidgumberg/gtk,jadahl/gtk,ahodesuka/gtk,alexlarsson/gtk,Lyude/gtk-,ahodesuka/gtk,grubersjoe/adwaita,Adamovskiy/gtk,davidgumberg/gtk,Lyude/gtk-,davidgumberg/gtk,alexlarsson/gtk,ahodesuka/gtk,ahodesuka/gtk,Adamovskiy/gtk,Adamovskiy/gtk,jadahl/gtk,Adamovskiy/gtk,alexlarsson/gtk,ahodesuka/gtk,jigpu/gtk,jadahl/gtk,alexlarsson/gtk,chergert/gtk,chergert/gtk,grubersjoe/adwaita,Lyude/gtk-,ahodesuka/gtk,jigpu/gtk,jigpu/gtk,alexlarsson/gtk,chergert/gtk,Adamovskiy/gtk,ahodesuka/gtk,jadahl/gtk,jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,Lyude/gtk-,Adamovskiy/gtk,ahodesuka/gtk,Adamovskiy/gtk,Lyude/gtk-,chergert/gtk,grubersjoe/adwaita,chergert/gtk,alexlarsson/gtk,jigpu/gtk,jigpu/gtk