title: How insert view (child) into div of parent layout? Vaadin 10 / Flow
body: <p>I have a component implementing <strong>RouterLayout</strong> like this:</p>
  
  <pre><code>@Tag("side-menu")
  @HtmlImport(value = "src/components/side-menu.html")
  public class SideMenu extends PolymerTemplate&lt;TemplateModel&gt; implements RouterLayout {
  
      @Id("menu")
      private PaperListBox listBox = new PaperListBox();
  
      public SideMenu() {
  
          listBox.addMenu(new PaperItem("tutorial", TutorialView.class));
          listBox.addMenu(new PaperItem("icons", IconsView.class));
  
      }
  }
  </code></pre>
  
  <p>I route a view child of the parent layout</p>
  
  <pre><code>@Route(value=IconsView.VIEW_ROUTE, layout = SideMenu.class)
  public class IconsView extends Div {
  
      public static final String VIEW_ROUTE = "icons";
  
      public IconsView() {
          add(new Label("ICONS VIEW"));
      }
  
  }
  </code></pre>
  
  <p>But the result overwrote all the content of <strong>side-menu.html</strong> file.</p>
  
  <p><strong>side-menu.html</strong> template base format</p>
  
  <pre><code>&lt;side-menu&gt;
      &lt;div&gt;App Name&lt;/div&gt;
      &lt;div id="menu"&gt;&lt;/div&gt;
      &lt;div id=contenido&gt;&lt;!-- I want to show here my view Icons --&gt;&lt;/div&gt;
  &lt;/side-menu&gt;
  </code></pre>
  
  <p>But the result it's</p>
  
  <pre><code>&lt;side-menu&gt;
      &lt;div&gt;
         &lt;label&gt;ICONOS VIEW&lt;/label&gt;
      &lt;/div&gt;
  &lt;/side-menu&gt;
  </code></pre>
  
  <p>The expected result is:</p>
  
  <pre><code>&lt;side-menu&gt;
      &lt;div&gt;App Name&lt;/div&gt;
      &lt;div id="menu"&gt;&lt;/div&gt;
      &lt;div id=contenido&gt;
         &lt;div&gt;
            &lt;label&gt;ICONOS VIEW&lt;/label&gt;
          &lt;/div&gt;
      &lt;/div&gt;
  &lt;/side-menu&gt;
  </code></pre>