EventIcon: Make the child window of the event box invisible

We do use the EventBox only to receive events, hence the
window that the even box creates should be a GDK_INPUT_ONLY
window, which means that it is invisible and only serves to
receive events [1].

[1] http://developer.gnome.org/gtk3/3.4/GtkEventBox.html#gtk-event-box-set-visible-window

Signed-off-by: Simon Schampijer <simon@laptop.org>
Reviewed-by: Benjamin Berg <benzea@sugarlabs.org>
Acked-by: Manuel Quiñones <manuq@laptop.org>

src/sugar3/graphics/icon.py

@@ -561,6 +561,8 @@ class EventIcon(Gtk.EventBox):
     def __init__(self, **kwargs):
         Gtk.EventBox.__init__(self)
 
+        self.set_visible_window(False)
+
         self._icon = Icon()
         for key, value in kwargs.iteritems():
             self._icon.set_property(key, value)