Sets surface as the icon for a given drag.
GTK+ retains references for the arguments, and will release them when they are no longer needed.
To position the surface relative to the mouse, use set_device_offset on surface. The mouse
cursor will be positioned at the (0,0) coordinate of the surface.
| context |
the context for a drag (This must be called with a context for the source side of a drag) |
| surface |
the surface to use as icon |