Allow registration of different backend login forms
The TYPO3 backend login form has hard-coded support for username/password and OpenID. There is no way to provide an additional login form, and no way to dynamically add new elements to the backend login form.
It should be possible to hook into the backend login form rendering process and add custom forms.
The existing forms (username/password) and openid should be moved to the new hook system.
This would also allow to disable the username/password form completely and e.g. show the OpenID input field only (#19618).