feature #2193 Make form groups collapsible (alterphp)

This PR was submitted for the master branch but it was squashed and merged into the 1.x branch instead (closes #2193).

Discussion
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Make form groups collapsible

I send it to master, but ... it basically can go the 1.x branch as well.

Commits
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6ee3612 Make form groups collapsible

doc/book/edit-new-configuration.rst

@@ -751,6 +751,17 @@ very advanced layouts.
     This solves most of the issues, but sometimes you might be forced to also
     reorder the form group positions.
 
+.. tip::
+
+    Taking advantage of Bootstrap capabilities, you can make form groups **collapsible**:
+
+    .. code-block:: yaml
+
+        # ...
+        - { type: 'group', collapsible: true, expanded: false }
+
+    An icon button in the title bar toggles the content. **expanded** option defines the state at load (default: *true*).
+
 Form Tabs
 .........
 

src/Resources/views/form/bootstrap_3_layout.html.twig

@@ -545,10 +545,15 @@
                             {% endif %}
                             {{ group_config.label|trans(domain = _translation_domain)|raw }}
                         </h3>
+                        {% if group_config.collapsible|default(false) %}
+                          <a class="pull-right" role="button" data-toggle="collapse" href="#box-body-{{ group_name }}">
+                            <i class="fa fa-expand"></i>
+                          </a>
+                        {% endif %}
                     </div>
                 {% endif %}
 
-                <div class="box-body">
+                <div class="box-body{% if group_config.collapsible|default(false) %} collapse{% if group_config.expanded|default(false) %} in{% endif %}{% endif %}" id="box-body-{{ group_name }}">
                     {% if group_config.help|default(false) %}
                         <p class="help-block">{{ group_config.help|trans(domain = _translation_domain)|nl2br|raw }}</p>
                     {% endif %}