[OAuth] Access token expiration convenience functionality

Access tokens (which are acquired with OAuth) typically have an expiration time, after which they are no longer valid.

The QAbstractOAuth2 class provides expiration property, a QDateTime indicating the expiration time.
Once the token has expired, the user needs to either refresh it with refresh_token, or restart the authorization flow.

This leaves it up to every application to handle. QAbstractOAuth2 class could provide convenience for this:

• aboutToExpire() signal
• automatic refresh if refresh token is available
• setter for the refresh threshold (how long before expiration to refresh)