Add possibility to hide empty value in customfield group view rendering

A new parameter is defined :

```
chill_custom_fields:
    show_empty_values_in_views = true|false
```

A new method is added to CustomFieldInterface: `isEmptyValue`. To ease
the dev of new classes, an AbstractCustomField class is created, which
implements the most commons function (currently, only isEmptyValue).

A new Twig Filter is added: `chill_custom_field_is_empty`

The twig filter `chill_custom_fields_group_widget` has a new possibility
in array option : `show_empty`. Default to
chill_custom_fields.show_empty_values_in_view. May be forced by
true/false.

Resources/views/CustomFieldsGroup/render.html.twig

@@ -2,7 +2,9 @@
     {% if customField.type == 'title' %}
         {{ chill_custom_field_widget(cFData , customField) }}
     {% else %}
+        {%- if show_empty == true or (chill_custom_field_is_empty(customField, cFData) == false) -%}
         <dt>{{ chill_custom_field_label(customField) }}</dt>
         <dd>{{ chill_custom_field_widget(cFData , customField) }}</dd>
-    {% endif %}
+        {%- endif -%}
+    {%- endif -%}
 {% endfor %}