function onOpen() {
  var spreadsheet = SpreadsheetApp.getActive();
  var menuItems = [
    {name: 'by label', functionName: 'fShowUI'}
  ];
  spreadsheet.addMenu('Gmail export', menuItems);
}

function fShowUI() {
  var myapp = UiApp.createApplication().setTitle('Google Mail - Label export');
  var mypanel = myapp.createVerticalPanel();
  var mylist = myapp.createListBox().setId('mylist');
  var mybutton = myapp.createButton('Select label').setId('mybutton');

  var labels = GmailApp.getUserLabels();
  for (var i = 0; i < labels.length; i++) {
    mylist.addItem(labels[i].getName())
  }

  callbackHandler = myapp.createServerHandler("fButtonConfirm");
  callbackHandler.addCallbackElement(mypanel);
  mybutton.addClickHandler(callbackHandler);

  mypanel.add(mylist);
  mypanel.add(mybutton);
  myapp.add(mypanel);

  /* Return myapp to display the UiInstance object and all elements associated with it. */
  var spreadsheet = SpreadsheetApp.getActive();
  spreadsheet.show(myapp);
}

function fButtonConfirm(e) {
  var spreadsheet = SpreadsheetApp.getActive();
  spreadsheet.toast(e.parameter.mylist);
}