Reduce bitrate slider sensitivity

[veganomy]

The bitrate slider seems to be too sensitive due to it's large range from 6 kbps to 512 kbps. And on top of that, floating values makes it too cumbersome to set a specific value bitrate. I always had to set ballpark values due to this.

An integer slider would be better. You can reduce the top range too. Because for opus codec, the transparency reaches around 160 kbps. Beyond that, the codec only adds filler data.