Refactor QAuthenticator

QAuthenticator is fragile and tries to do too many things in one class.
It should be split up something like this:

QAuthenticator - keep the public class, but just become a value type for passing credentials around. (should become implicitly sharable again)
QAuthenticationProvider - abstract base class with functions for calculating authentication responses
QAuthenticationProviderBasic - the "basic" scheme of http (password encoded but not encrypted or hashed)
QAuthenticationProviderPlaintext - password sent in the clear (e.g. ftp)
QAuthenticationProviderDigest - the "digest" scheme of http
QAuthenticationProviderNtlm - MS windows authentication

Any state would be part of the authentication provider instance.
Places where QAuthenticatorPrivate is being used, the authentication provider should be used instead with a clean interface.
The QAuthenticationProvider hierarchy should initially be internal

(this task is a placeholder for discussion)