[libcommhistory] Implement initial MMS receive handling

Here's the first part of handling incoming MMS via MMS engine. Other pieces are still being finished up in separate branches on top, but I'll submit these later:
 * Manually triggered receive/retry
 * Cancelling outstanding messages when deleted
 * Resetting event states on startup
 * Some robustness cases (e.g. MMS engine disappearing from dbus)
 * Notifications for error and "manual download needed" cases
 * Read reports
 * Send

Requires https://github.com/nemomobile/libcommhistory/pull/75