Add multiple sources in Demo

[Javier Godoy]

Commons demo supports multiple sources.

It would help if TestLoginLayoutView also displays the source for TestLoginLayout

ExtendedLoginAddon/src/test/java/com/flowingcode/vaadin/addons/extendedlogin/TestLoginLayoutView.java

Lines 42 to 43 in e5c7765

 // show-source @Route("value = "extended-login/login-layout-demo", layout = TestLoginLayout.class) 
 public class TestLoginLayoutView extends Div {