Feature: #88921 - New PSR-14 events in the PageLayoutView class

See Issue #88921

Description

Two new PSR-14 events have been added to the PageLayoutView class. Those events can be used to add content into any column of a BackendLayout. You can use this for example to show some content in a column without a colPos assigned.

The event BeforeSectionMarkupGeneratedEvent can be used to add content above the content elements of the column. The event AfterSectionMarkupGeneratedEvent can be used to add content below the content elements of the column.

You can use business logic to show content in specific columns. E.g. for displaying content only in columns without any colPos in the BackendLayout configuration.

Example how to register the event listener in your own extension:

EXT:my_extension/Configuration/Services.yaml

services:
  Vendor\MyExtension\Backend\View\PageLayoutViewDrawEmptyColposContent:
    tags:
      - name: event.listener
        identifier: 'myColposListener'
        before: 'backend-empty-colpos'
        event:  TYPO3\CMS\Backend\View\Event\AfterSectionMarkupGeneratedEvent

With before and after, you can make sure your own listener is executed before or after the given identifiers.

EXT:my_extension/Classes/Backend/View/PageLayoutViewDrawEmptyColposContent.php

<?php
namespace Vendor\MyExtension\Backend\View;

class PageLayoutViewDrawEmptyColposContent
{
   public function __invoke(AfterSectionMarkupGeneratedEvent $event): void
   {
      if (
          !isset($event->getColumnConfig()['colPos'])
          || trim($event->getColumnConfig()['colPos']) === ''
      ) {
         $content = $event->getContent();
         $content .= <<<EOD
            <div class="t3-page-ce-wrapper">
               <div class="t3-page-ce">
                  <div class="t3-page-ce-header">Empty colpos</div>
                  <div class="t3-page-ce-body">
                     <div class="t3-page-ce-body-inner">
                        <div class="row">
                           <div class="col-xs-12">
                              This column has no "colPos". This is only for display Purposes.
                           </div>
                        </div>
                     </div>
                  </div>
               </div>
            </div>
         EOD;

         $event->setStopRendering(true);
         $event->setContent($content);
      }
   }
}

With the $event->setStopRendering() method, you can make sure that no other listeners are triggered after the current listener.