Add blocks before and after main navigation

This commit adds two blocks before and after main navigation.

I will use this to add :

- locale switcher for admin
- store switcher
.....

Resources/views/standard_layout.html.twig

@@ -116,6 +116,8 @@ file that was distributed with this source code.
                             </a>
                         {% endblock %}
 
+                        {% block sonata_nav_menu_before %}{% endblock %}
+
                         {% block sonata_nav_menu %}
                             <div class="nav-collapse">
                                 <ul class="nav">
@@ -168,6 +170,9 @@ file that was distributed with this source code.
 
                             </div>
                         {% endblock %}
+                        
+                        {% block sonata_nav_menu_after %}{% endblock %}
+                        
                     {% endif %}
                 </div>
             </div>