code: protected boolean onBeforeActivate(Widget newTabHeader, Widget oldTabHeader,
                                         Widget newTabContent, Widget oldTabContent) {
          return true;
      }
docs: If the tabs are currently collapsed, oldTabHeader and oldTabContent will be null.
  <br> If the tabs are collapsing, newTabHeader and newTabContent will be null.
  
  @param newTabHeader - JQMButton or JQMListItem
  @param oldTabHeader - JQMButton or JQMListItem
  @param newTabContent - Widget
  @param oldTabContent - Widget