title: Converter called twice with list controller
body: <p>I binded the selected item of a list with a label widget. I also added a converter to transform my data. Although the binding is unidrectionnal (<a href="http://www.qooxdoo.org/5.0.1/apiviewer/#qx.data.SingleValueBinding~bind" rel="nofollow noreferrer">http://www.qooxdoo.org/5.0.1/apiviewer/#qx.data.SingleValueBinding~bind</a>), the converter seems to be called twice.</p>
  
  <p>Here is a <a href="http://www.qooxdoo.org/devel/playground/#%7B%22code%22%3A%22var%2520rawData%2520%253D%2520%255B%255D%253B%250D%250Afor%2520(var%2520i%2520%253D%25200%253B%2520i%2520%253C%252010%253B%2520i%252B%252B)%2520%257B%250D%250A%2520%2520rawData.push(%2522Item%2520%2522%2520%252B%2520i)%253B%250D%250A%257D%250D%250Avar%2520data%2520%253D%2520new%2520qx.data.Array(rawData)%253B%250D%250A%2520%2520%2520%2520%2520%2520%250D%250Avar%2520list%2520%253D%2520new%2520qx.ui.form.List()%253B%250D%250Avar%2520label%2520%253D%2520new%2520qx.ui.basic.Label().set(%257Bmargin%253A20%257D)%253B%250D%250A%250D%250Avar%2520controller%2520%253D%2520new%2520qx.data.controller.List(data%252C%2520list)%253B%250D%250Acontroller.bind(%2522selection%255B0%255D%2522%252C%2520label%252C%2520%2522value%2522%252C%257B%250D%250A%2520%2520converter%2520%253A%2520function(data%252Cmodel%252Csource%252Ctarget)%257B%250D%250A%2520%2520%2520%2520console.log(data)%253B%250D%250A%2520%2520%2520%2520return%2520data%253B%250D%250A%2520%2520%257D%250D%250A%257D)%253B%250D%250A%250D%250Avar%2520doc%2520%253D%2520this.getRoot()%253B%250D%250A%250D%250Avar%2520container%2520%253D%2520new%2520qx.ui.container.Composite(new%2520qx.ui.layout.VBox())%253B%250D%250Acontainer.add(list%252C%257Bflex%253A1%257D)%253B%250D%250Acontainer.add(label)%253B%250D%250A%250D%250Adoc.add(container%252C%257Bedge%253A0%257D)%253B%250D%250A%250D%250A%22%2C%20%22mode%22%3A%22ria%22%7D" rel="nofollow noreferrer">code snippet</a>. Select any element in the list and check the browser's console.</p>
  
  <p>Shoudln't it be called only once?</p>
  
  <p>I also found this related thread on the Gitter chat. The user's problem is solved but it doesn't explain why the converter is called twice with a unidirectionnal binding :
  <a href="https://gitter.im/qooxdoo/qooxdoo?at=579b01dc1d6bf8244e2b5f71" rel="nofollow noreferrer">https://gitter.im/qooxdoo/qooxdoo?at=579b01dc1d6bf8244e2b5f71</a></p>