Allow to use field_group label in patterns.

Author: Edouard Cunibil

modules/ui_patterns_field_group/src/Plugin/UiPatterns/Source/FieldgroupSource.php

@@ -39,6 +39,7 @@ public function getSourceFields() {
         $sources[] = $this->getSourceField($group_name, $group->label);
       }
     }
+    $sources[] = $this->getSourceField('_label', 'Group label');
 
     return $sources;
   }

modules/ui_patterns_field_group/src/Plugin/field_group/FieldGroupFormatter/PatternFormatter.php

@@ -131,7 +131,12 @@ protected function preRenderGroup(array &$element, $group_name, array $rendering
       // Move content into their fields.
       foreach ($group->format_settings['pattern_mapping'] as $field) {
         if ($field['plugin'] == 'fieldgroup') {
-          $this->preRenderGroup($element[$field['source']], $field['source'], $rendering_object);
+          if ($field['source'] === '_label') {
+            $element[$field['source']] = ['#markup' => $group->label];
+          }
+          else {
+            $this->preRenderGroup($element[$field['source']], $field['source'], $rendering_object);
+          }
         }
         $element['#fields'][$field['destination']][$field['source']] = $element[$field['source']];
       }