Added JavaDocs to Header related classes

java/com.sap.sailing.gwt.ui/src/main/java/com/sap/sailing/gwt/home/mobile/partials/header/Header.java

@@ -26,6 +26,9 @@ import com.sap.sse.gwt.client.LinkUtil;
 import com.sap.sse.security.ui.authentication.AuthenticationContextEvent;
 import com.sap.sse.security.ui.authentication.AuthenticationSignOutRequestEvent;
 
+/**
+ * Mobile page header with SAP logo title and a dropdown menu (burger button) on the right.
+ */
 public class Header extends Composite {
 
     // @UiField TextBox searchText;

java/com.sap.sailing.gwt.ui/src/main/java/com/sap/sailing/gwt/home/mobile/partials/header/HeaderNavigationItem.java

@@ -11,6 +11,9 @@ import com.google.gwt.uibinder.client.UiBinder;
 import com.google.gwt.uibinder.client.UiField;
 import com.google.gwt.user.client.ui.Widget;
 
+/**
+ * Represents an entry in the header's popup menu.
+ */
 public class HeaderNavigationItem extends Widget implements HasClickHandlers {
 
     @UiField