allow prefilling login

I want to be able to send deep links into the gallery, that also prefill the username if the user is not logged in, sth like http://localhost/core/index.php?redirect_url=%2Findex.php%2Fapps%2Fgallery&user=guest

The guest account has quota zero and can only access the files and pictures apps.

@LukasReschke security nightmare?
@jancborchardt usability improvement?