title: Extjs keeping current select items in combo and adding new items
body: <p>I have a Role combo box which already have 3 item (loaded User Model). Now I need to add the 4th item to the selection but it doesn't keep the current items, and when I add the 4th one, it will be the only item selected in combo.</p>
  
  <p>Is there anyway to implement this behavior ? 
      My grid record is selected like this</p>
  
  <pre><code> doSelectUser: function(grid, record) {
          var me = this;
          var selectedRoleVals = record.get('roles');
          me.getUserForm().loadRecord(record);
  </code></pre>
  
  <p>And then I click the combo box in User Form:</p>
  
  <pre><code> manageusers userform combobox[name=roles]': {
                  select: this.doSelectRole
              }
  </code></pre>
  
  <p>combo handler</p>
  
  <pre><code>doSelectRole: function(combo, records) {
          var me = 
          var rec = records[0];
  </code></pre>