Use gtk_window_get_position to get (x, y)

The `x` and `y` from `GdkConfigureEvent` can be affected by the window
decorations. This offset is not consistent; sometimes it is affected,
sometimes it is not.

The `gtk_window_get_position()` function produces more consistent results.
If this acts up, make sure the window has `GDK_GRAVITY_STATIC`.
Start drag and drop

So far just drop. Use Caja or Nautilus to experiment.

This sets the icon view as a drag and drop destination for text and URI
targets.