Bug 1050381 - Support accessibility edge gestures.

Ideally this should really look like:

```
SheetsTransition.begin(direction);
if (direction === 'ltr') {
  SheetsTransition.snapBack();
  StackManager.goPrev();
} else {
  SheetsTransition.snapForward();
  StackManager.goNext();
}
```
We need to default to speed = 1 in `SheetsTransition._snapAway`, but what else is breaking if we do this?