Allow to use plugin icon via identifier

The icon is still a placeholder.
Still we provide the identifier to already use it in sitepackages.
That way the extension can exchange the file, but foreign configuration
can stay.

Relates: #8092

ext_localconf.php

@@ -26,15 +26,14 @@ call_user_func(
             [\Wrm\Events\Controller\DateController::class => 'show']
         );
 
-        /*
         $iconRegistry = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(\TYPO3\CMS\Core\Imaging\IconRegistry::class);
-
         $iconRegistry->registerIcon(
             'events-plugin',
             \TYPO3\CMS\Core\Imaging\IconProvider\SvgIconProvider::class,
             ['source' => 'EXT:events/Resources/Public/Icons/user_plugin_events.svg']
         );
 
+        /*
         // wizards
         \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPageTSConfig(
             'mod {