Play local ringback tone according to oFono signalling

Since its initial check-in, tp-ring contains lots of dead code related to the "user connection" during a call, i.e. whether or not an audio path between the phone and the network is open. This code was meant to be used with an older, proprietary modem API. Our modem API (oFono with underlying RIL) does not provide that level of detail. The only case that we can handle is a missing "alerting" tone of an MO call (a.k.a. ringback tone), so I simplified the code.

tp-ring now registers itself as a VoicecallAgent towards oFono, unfortunately adding a significant amount of D-Bus handling code (tp-ring's "modem interface" part has nice abstractions for calls from tp to oFono, but nothing for the opposite direction which we need here). This pull request is meant to be used with oFono pull request https://github.com/nemomobile-packages/ofono/pull/214 (but it's safe to use with any other oFono version, too, there's just no ringback functionality).