PyGObject: modify drag icon in TreeView

[<c buhtz posteo jp>]

I want to use the "drag-begin" event to show an individual drag-icon.
<https://lazka.github.io/pgi-docs/#Gtk-3.0/classes/Widget.html#Gtk.Widget.signals.drag_begin>

But it has no effect in my Gtk.TreeView
 - I tested with connect() and connect_after()
 - the events are called (tested by print-call)
 - I created a GdkPixbuf.Pixbuf.new_from_file() which returns a valid
   object and not None

Any idea?