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.
Reword with `an array of objects` and describe those objects, please.