feat: set a default id in the demo content

Author: javier-godoy

src/main/java/com/flowingcode/vaadin/addons/demo/TabbedDemo.java

@@ -122,6 +122,9 @@ public void addDemo(Component demo) {
    * @param sourceCodeUrl the url of the demo, <b>null</b> to not show source code section.
    */
   public void addDemo(Component demo, String label, String sourceCodeUrl) {
+    if (!demo.getId().isPresent()) {
+      demo.setId("content");
+    }
     Tab tab = new Tab(label);
     if (sourceCodeUrl != null) {
       demos.put(tab, new SplitLayoutDemo(demo, sourceCodeUrl));