Allow the network platform plugin to hook into QAuthenticator

Many platforms provide special authentication dialogs, authentication cache or single-sign-on (SSO) features. Examples of those are:

• Maemo (Harmattan)
• KDE (integration with KWallet)

There should be a hook in the network platform plugin that gets called by QAuthenticator whenever an authentication is required – e.g., when an authenticationRequired(QAuthenticator*,SomeOtherClass) or proxyAuthenticationRequired(QAuthenticator*,QNetworkProxy) signal is about to be emitted.

The platform plugin would then fill in the required fields, without the signal being emitted and the user code needing to handle it.

In case of authentication failed, the network platform plugin must be notified too (e.g., include the failure count for the same "location" since the last pass). For this purpose, and since the "location" that the authentication refers to can vary, a special payload class may be necessary for calling the platform plugin.