title: Editable ComboBox with binding to value not in list
body: <p>I have editable combobox where not always the preferred item is in the drop-down list. </p>
  
  <p>I would like to have the possibility of manually entering text in the textbox which is propagated to the string bound to SelectedValue.</p>
  
  <p>Right now the string bound to SelectedValue is only updated if the entered value is on of the ones in the ComboBox items.</p>
  
  <p>How do I allow custom values not available in the ComboBox list to be manually entered and properly propagated to bound value?</p>