title: Upload without upload button and browse button
body: <p>I'm trying to use the Upload component to browse and upload a file i. What I want is only upload-functionality, but not the upload button and the browse button.I want to add a button to the page when a user click it the browse file window is opened so he can choose a file and upload it
  I wrote this :</p>
  
  <pre><code>@Override
  protected void init(VaadinRequest request) {
      final VerticalLayout layout = new VerticalLayout();
      layout.setMargin(true);
      setContent(layout);
      final Upload sample = new Upload(null, null);
      Button button = new Button("Click Me");
      sample.setImmediate(true);
              sample.setButtonCaption(null);
              System.out.println(sample.isVisible());
      button.addClickListener(new Button.ClickListener() {
          public void buttonClick(ClickEvent event) {
              layout.addComponent(new Label("Thank you for clicking"));
              sample.setVisible(true);
              layout.addComponent(sample);
          }
      });
      layout.addComponent(button);
    }
  
   }
  </code></pre>