Use render element container instead of fieldset for HorizontalLayout and VerticalLayout

This avoids additional padding/margin or borders (depending on the
theme).

Author: Dominic Tubach

src/Form/Layout/HorizontalLayoutArrayFactory.php

@@ -32,7 +32,7 @@ public function supportsDefinition(DefinitionInterface $definition): bool {
 
   protected function createBasicFormArray(LayoutDefinition $definition): array {
     return [
-      '#type' => 'fieldset',
+      '#type' => 'container',
       '#attributes' => ['class' => ['container-inline']],
     ];
   }

src/Form/Layout/VerticalLayoutArrayFactory.php

@@ -31,7 +31,7 @@ public function supportsDefinition(DefinitionInterface $definition): bool {
   }
 
   protected function createBasicFormArray(LayoutDefinition $definition): array {
-    return ['#type' => 'fieldset'];
+    return ['#type' => 'container'];
   }
 
 }