Most editors use the 'New content element wizard' to add a new content element to a page. This can be configured in page TSConfig. Because we need to add some generated paths the configuration is added
to ext_localconf.php instead of a separate file.

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPageTSConfig(
  'mod {
   wizards.newContentElement.wizardItems.extra {
      header = LLL:EXT:typo3coder/Resources/Private/Language/newContentElements.xlf:extra
      elements {
        intro {
          icon = ' . \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::extRelPath($_EXTKEY) . 'Resources/Public/Backend/Images/intro.png
          title = LLL:EXT:typo3coder/Resources/Private/Language/newContentElements.xlf:extra_intro_title
          description = LLL:EXT:typo3coder/Resources/Private/Language/newContentElements.xlf:extra_intro_description
          tt_content_defValues {
            CType = intro
          }
        }
        slider {
          icon = ' . \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::extRelPath($_EXTKEY) . 'Resources/Public/Backend/Images/slider.png
          title = LLL:EXT:typo3coder/Resources/Private/Language/newContentElements.xlf:extra_slider_title
          description = LLL:EXT:typo3coder/Resources/Private/Language/newContentElements.xlf:extra_slider_description
          tt_content_defValues {
            CType = slider
          }
        }
      }
      show = *
    }
  }'
);

There is now a new tab 'extra' with two elements 'intro' and 'slider'. Each has a title, description and icon.