bearer code: Add methods for retrieving the bearer type family (2G / 3G / 4G)

The current QNetworkConfiguration::bearerType() method returns the type in great detail, e.g. QNetworkConfiguration::BearerCDMA2000, QNetworkConfiguration::BearerWCDMA or QNetworkConfiguration::BearerHSPA. IMO the standard app developer is only interested (if at all) whether he is on Wifi, 2G, 3G or 4G. There should be a method that just returns the latter, which should be enough for most use cases.