[gharlan]

The in-use checks of media and media categories become typed extension points, and articles get the same check, so addons can block the deletion of an article they still reference (e.g. YForm relations).

Changes

• New Util\UsageList: extensions add usages via add($group, $label, ?$url) (all plain text); the core renders and escapes the list in one place. Extensions no longer build HTML snippets.
• Extension points (readonly, carrying the item and $usages):
  • MEDIA_IS_IN_USE → MediaPool\ExtensionPoint\MediaUsageCheck
  • MEDIA_CATEGORY_IS_IN_USE → MediaPool\ExtensionPoint\MediaCategoryUsageCheck
  • new: Content\ExtensionPoint\ArticleUsageCheck
• MediaPool::mediaIsInUse() and MediaCategoryHandler::isInUse() are replaced by findUsages() returning the UsageList; ArticleHandler::findUsages() is added. The rector mappings to the old methods are removed, since the signatures changed.
• Meta info: MetaInfo\MediaUsage becomes MetaInfo\MetaUsage with addMediaUsages() and addArticleUsages(), called directly (no extension point). Articles referenced in article picker meta fields can no longer be deleted; the article itself and the category it is the start article of are skipped.
• Deleting a referenced article shows the grouped usage list with links (like the media pool) instead of a plain list of names.
• Usage links are plain links now. openPage() only works in the media pool popup, so mediapool.js redirects usage links leading out of the media pool to the main window only there.
• Lang keys: minfo_media_in_use_* → minfo_usage_*, pool_file_in_use_articles → usage_articles_content.

Testing

• Unit tests for the rendering/escaping of UsageList and for an extension blocking the deletion via ArticleUsageCheck.
• In the browser with a temporary meta schema (article picker + media picker on articles): deleting an article referenced in the meta fields fails with the usage list, an article referencing itself can be deleted, deleting a referenced medium in the media pool popup shows the list, and its link opens the article in the main window and closes the popup.