Bug 1011482 - add value and compositeKey for alt

Alternative keys can be represent as

- `'€£'` when all alternative is a single character,
- `'€ R$'` when at least one alternative is a compositeKey and
- `[{value: '€', compositeKey: '\\Euro'}, {value: '£', compositeKey:
  '\\textsterling'}]` when at least one alternative is a
  compositeKey different from the value/label to be show at the
  keyboard.
You can make this shorter by doing:
```javascript
var alt = alternatives[i];
alternatives[i] = { value: alt };
if (alt.length > 1) {
  alternatives[i].compositeKey = alt;
}
```

Or even:
```javascript
var alt = alternatives[i];
alternatives[i] = { value: alt, compositeKey: alt.length > 1 ? alt : undefined };
```
I prefer the first form.