Separate the credential dialog from their fetch #3350

Separate the credential dialog from their fetch #3350

This moves the responsibility of asking the user or not for
credentials from the Credentials classes back to the AccountState.
fetch() now only extract credentials from the keychain, reports
the result to the AccountState which then decides if askFromUser()
should be called or not. The result is once more reported to the
AccounState.

This also replaces the HttpCredentials::queryPassword virtual
which now lets HttpCredentialsGui and HttpCredentialsText do it
the way that they prefer.