Feature: #84045 - new AdminPanel module API

See forge#84045

Description

Extending the Admin Panel was only partially possible in earlier TYPO3 versions by using a hook that provided the possibility to add pure content (no new modules) as plain HTML.

A new API has been introduced, providing more flexible options to add custom modules to the admin panel or replace and deactivate existing ones.

Impact

Custom admin panel modules can now be registered via $GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['adminpanel']['modules'].

$GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['adminpanel']['modules']['yourmodulename'] = [
    'module' => \Vendor\Package\AdminPanel\YourModule::class,
    'after' => ['preview']
]
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To implement a custom module your module class has to implement the \TYPO3\CMS\Adminpanel\Modules\AdminPanelModuleInterface.

Be aware that the \TYPO3\CMS\Adminpanel\Modules\AdminPanelModuleInterface is not final yet and may change until v9 LTS.