The mention @ WYSIWYG editor shortcut is not always registered

This doesn't happen very often in an XWiki standard instance, but it happens very often in XWiki instances build for functional tests. The problem is a race condition between:

• the code that loads the WYSIWYG editor (CKEditor)
• the mention JSX (always on this wiki) that modifies the CKEditor configuration

Everything's fine if the configuration is updated before the CKEditor instance is ready, but that's not always the case. There's nothing in the mention JSX to force it to be executed before the CKEditor instance is ready.