title: Wicket fails to find markup file
body: <p>I am building a custom <code>FormComponentPanel</code> for Wicket. This is done in its own Maven project.
  This project is later added to my webapp as a dependecy. At the moment my custom panel does not
  contain extra functionality. I have the following files inside the same package (under src/main/java/package).</p>
  
  <p>CustomFormPanel.java:</p>
  
  <pre><code>class CustomFormPanel extends FormComponentPanel&lt;String&gt; {
  
      public CustomFormPanel(final String id) {
          super(id);
      }
  
  }
  </code></pre>
  
  <p>CustomFormPanel.html:</p>
  
  <pre><code>&lt;wicket:panel&gt;
  
  &lt;/wicket:panel&gt;
  </code></pre>
  
  <p>I use this component as following:</p>
  
  <p>CustomPage.java:</p>
  
  <pre><code>public class CustomPage extends WebPage {
  
      private final StatelessForm&lt;Void&gt; form;
  
      private FormComponentPanel&lt;String&gt; customPanel;
  
      public CustomPage(final PageParameters params) {
          super(params);
  
          customPanel = new CustomFormPanel("customPanel");
  
          form = new StatelessForm&lt;Void&gt;("form") {
  
              @Override
              public void onSubmit() {
                  final String param = customPanel.getModelObject();
              }
  
          };
      }
  
      @Override
      public void onInitialize() {
          super.onInitialize();
  
          form.add(customPanel);
          add(form);
      }
  
  }
  </code></pre>
  
  <p>CustomPage.html:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;!DOCTYPE html&gt;
  &lt;html xmlns="http://www.w3.org/1999/xhtml" xmlns:wicket="http://wicket.apache.org"&gt;
  &lt;head&gt;
  &lt;meta http-equiv="Content-Type" content="text/html; charset=UTF-8"&gt;
  &lt;/head&gt;
  &lt;body&gt;
      &lt;form wicket:id="form"&gt;
          &lt;wicket:container wicket:id="customPanel"&gt;&lt;/wicket:container&gt;
          &lt;input type="submit" value="Job erstellen" /&gt;
      &lt;/form&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>If I navigate to this site in my browser, I get following error message:</p>
  
  <pre><code>Last cause: Failed to find markup file associated. CustomFormPanel: [CustomFormPanel [Component id = customPanel]]
  </code></pre>