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.
The original problem is that showing a popup not originated
from the main settings window while it's focused won't be
shown in front to the user.
This try not to highjack the user's attention of the user
by showing a notification when checking the connection for
valid credentials, and require the user to sign in through
the UI. There are still issues with showing that popup from
the tray icon, but the user will most likely be looking for
the popup in that case. The new sign in button directly in
the settings account works properly.
