feat(demo): add extending layout demo

Author: mlopezFC

src/test/java/com/flowingcode/vaadin/addons/applayout/ExtendingAppLayoutDemo.java

@@ -0,0 +1,24 @@
+package com.flowingcode.vaadin.addons.applayout;
+
+import com.flowingcode.vaadin.addons.demo.DemoSource;
+import com.vaadin.flow.component.html.Div;
+import com.vaadin.flow.component.html.IFrame;
+import com.vaadin.flow.router.PageTitle;
+import com.vaadin.flow.router.Route;
+
+@DemoSource("/src/test/java/com/flowingcode/addons/applayout/CustomAppLayout.java")
+@DemoSource("/src/test/java/com/flowingcode/addons/applayout/SampleView.java")
+@PageTitle("Extending AppLayout Demo")
+@SuppressWarnings("serial")
+@Route(value = "applayout/extending-applayout-demo", layout = AppLayoutDemoView.class)
+public class ExtendingAppLayoutDemo extends Div {
+
+  public ExtendingAppLayoutDemo() {
+    setClassName("wrap-iframe");
+    IFrame iframe = new IFrame("/view");
+    iframe.setClassName("frame");
+    iframe.setSizeFull();
+    iframe.getElement().setAttribute("frameBorder", "0");
+    add(iframe);
+  }
+}