Investigate changes to QAuthenticator for allowing third-party authentication mechanisms

The network platform plugin may be able to add authentication mechanisms to what's provided in Qt. Alternatively, these can be provided by QtNetwork plugins too.

QAuthenticator's internals are far too tied to the parsing of HTTP requests, but QAuthenticator could reasonably be used for authenticating to IMAP as well (Note: need one more protocol for a rule-of-three conception)

This should allow overriding of QAuthenticator's methods too. For example, a Single-Sign-On (SSO) method could obtain a pre-made response from an external cache.