Support for GSM supplementary voice services

This MR adds support for GSM supplementary services like "call waiting", "call hold", "call transfer", "call deflection" and so on.

In order to support having multiple active calls at the same time, the MMBaseCall and MMIfaceModemVoice have been reworked, e.g. so that "in-call URCs" or "audio channel setup" are not performed at call object level, but at modem object level instead (because the would apply to all active calls).

The plugins that had voice support have also been updated and improved to be able to support multiple active calls, e.g. by making sure the call index is never ignored.