Stepper (selectionChange) vs (selectionChanged) and [selectedIndex]

#### Bug, feature request, or proposal:
```


Next


Previous (cancelled)


```
In the stepper control the (selectionChange) event cannot be used together with [selectedIndex], because changing the [selectedIndex] also triggers (selectionChange), so there is no way of cancelling the selection programatically, or implement other logic 'before' the change is executed.

Expected it to work as a standard html select control where:
- when changing the .selectedIndex the (change) event doesn't get automatically triggered
- the (change) event is triggered 'before' the change is finished, so it can be cancelled with event.preventDefault()

#### What is the expected behaviour?
- The current event should be renamed (selectionChange**d**) = 'after' the step has changed.
- Emit a new event (selectionChange) = 'before' the step has changed, with option to cancel the action, which doesn't get automatically triggered when selectedIndex changes.

#### What is the current behavior?
selectionChange = 'after' the step has changed, naming is confusing, gets triggered automatically with [selectedIndex]

#### What are the steps to reproduce?

1. Select step 2
2. Press Previous button (the action is cancelled programatically) - ok
3. Press step 1 header (step 1 is still selected, the action cannot be cancelled, selectedIndex remains on step 2)

StackBlitz: https://stackblitz.com/edit/angular-material2-issue-9lskal

#### What is the use-case or motivation for changing an existing behavior?
Be able to programatically enable/disable certain steps

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Material 5.1.0

#### Is there anything else we should know?

Inizia riproducendo il comportamento nello StackBlitz collegato, concentrandoti su mat-horizontal-stepper, selectedIndex e selectionChange. Traccia le differenze tra la navigazione dell’utente e le modifiche programmatiche a selectedIndex; il lavoro è completo quando la semantica degli eventi è distinta, l’azione precedente alla modifica può essere annullata e la notifica successiva alla modifica rimane disponibile.

Scritto dal modello di indicizzazione a partire dal testo della issue.