Speed rename

This fixes Bug https://bugs.launchpad.net/mixxx/+bug/1283190
I think Daniel is doing this because EngineBuffer means two different things when we say rate and speed:

See my comment on line 826:

```
            // The way we treat rate inside of EngineBuffer is actually a
            // description of "sample consumption rate" or percentage of samples
            // consumed relative to playing back the track at its native sample
            // rate and normal speed. pitch_adjust does not change the playback
            // rate.
            m_rate_old = rate = baserate * speed_adjust;
```