title: Formatter function in XML view returns error
body: <p>
  I try to format the visibility property according to a custom parameter.</p>
  
  <p>Element definition in the view:</p>
  
  <pre class="lang-js prettyprint-override"><code>&lt;CheckBox id="compensation0" selected="false" enabled="true" 
  visible="{formatter: 'my.util.Formatter.visible'}"  editable="true" select=""/&gt;
  </code></pre>
  
  <p>Formatter:</p>
  
  <pre class="lang-js prettyprint-override"><code>my.util.Formatter = {
      visible: function(){
          return true;
      }
  };
  </code></pre>
  
  <p>The error I get:</p>
  
  <p>UIComponent.js:6 Uncaught Error: "[object Object]" is of type object,
  expected boolean for property "visible" of 
  Element sap.m.CheckBox #__xmlview1--compensationColumn</p>
  
  <p>What did I do wrong?</p>
  
  <p>Thank you.</p>