title: extjs emptyText dissappears on user selection
body: <p>I have the following. My issue is that when the user selects a client the "--- Clients ---" disappears from the combo box. Is emptyText the right property to be using here. </p>
  
  <pre><code> {
                                  xtype:'combobox',
                                  name:'Clients',
                                  id:'Clients',
                                  displayField:'Name',
                                  emptyText:'-------- Clients --------',
                                  store:'Clients',
                                  queryMode:'local',
                                  mode:'local',
                                  allowBlank:true,
                                  valueField:'Id',
                                  width:200
                              },
  </code></pre>