[Bug] EV3 back button not exposed

**Describe the bug**
In Pybricks 2.0 the back/cancel button could not be used since it would exit the program.

But now we can do:

```
hub.system.set_stop_button(None)
```

And then `hub.buttons.pressed()` should include the back button. But it doesn't.

We could add a `Button.BACK`, but that would confuse things with NXT where it is `Button.DOWN`, so we'll probably stick with  `Button.LEFT_UP`.