title: format text of Label
body: <p>I have a label in a view that is controlled by a integer property, when the value is negative it displays with a minus sign, when the value is positive it does not. However, I would like the Label to display "+5", "-3" ...</p>
  
  <p>take the following code as an example</p>
  
  <pre><code>import javafx.beans.property.SimpleIntegerProperty
  import tornadofx.*
  
  class MyView : View() {
  
     val negProp = SimpleIntegerProperty(-3) // this prop is in a ItemViewModel
     val posProp = SimpleIntegerProperty(+4) // this prop is in a ItemViewModel
  
      override val root = hbox {
          label(negProp)      // shows - 3
          label(posProp)      // shows 4
      }
  }
  </code></pre>
  
  <p>Is there a way I can format the text once the property changes ? 
  Thank you.</p>