Usermanager: Add autogeneration of password

    The password of a newly added user account is now autogenerated
    if the backend supports it, the password field is left empty and
    the notification of the user is enabled. A corresponding note was
    added to the dialog to inform admins about this behaviour.

darcs-hash:20080901170041-2b4f5-e66173fe74395a1dab8e6deaf59541d258d4bacb.gz