Allows Saved Layout Library Labels to be edited

This is working nicely, thanks! We just need a few code clean ups and some test coverage.

1. +++ b/src/Entity/Layout.php
   @@ -45,6 +46,7 @@ use Drupal\Core\Url;
   + *     "edit-form" = "/admin/structure/layouts/manage/{layout}/edit",
   

   What about making this /admin/structure/layouts/manage/{layout}/configure for consistency with the form title?

2. +++ b/src/Form/LayoutEditForm.php
   @@ -0,0 +1,88 @@
   +  ¶
   

   Trailing whitespaces.

3. +++ b/src/Form/LayoutEditForm.php
   @@ -0,0 +1,88 @@
   +    $form['#title'] = $this
   +      ->getQuestion();
   ...
   +      '#markup' => $this
   +        ->getDescription(),
   ...
   +    $form[$this
   +      ->getFormName()] = array(
   

   Nit: do we need these newlines? :)

4. +++ b/src/Form/LayoutEditForm.php
   @@ -0,0 +1,88 @@
   +    $form['description'] = array(
   

   (here and below) Let's use the short array syntax ([]).

5. +++ b/src/Form/LayoutEditForm.php
   @@ -0,0 +1,88 @@
   +      '#title' => t('Label'),
   ...
   +      '#description' => t("Provide a label for this layout to help identify it in the administration pages."),
   

   $this->t

   Also, let's use single quotes consistently.

6. +++ b/src/Form/LayoutEditForm.php
   @@ -0,0 +1,88 @@
   +    return $this->t(' ');
   

   This will unnecessarily create a translatable string, let's just go with ''.

7. +++ b/src/Form/LayoutEditForm.php
   @@ -0,0 +1,88 @@
   +    //$this->entity->delete();
   

   Let's remove this :)

8. +++ b/src/Form/LayoutEditForm.php
   @@ -0,0 +1,88 @@
   +    $this->entity->save();
   

   Note for future me: this is working because the entity label is updated in the #after_build phase via a ::buildEntity() call.

9. +++ b/src/Form/LayoutEditForm.php
   @@ -0,0 +1,88 @@
   \ No newline at end of file
   

   Let's add an empty nee line here.