[mat-tree] Keyboard Interaction recommended by WAI-ARIA

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

#### What is the expected behavior?
Read expected behavior under "Keyboard interactions" at [w3.org/TR/wai-aria-practices/#TreeView](https://www.w3.org/TR/wai-aria-practices/#TreeView). Should use `TAB` to focus [mat-tree] and use arrows to navigate. Similar to radio button behaviour.

#### What is the current behavior?
`TAB` and `SHIFT + TAB` to navigate. `SPACE` or `ENTER` to open/close a node.

#### What are the steps to reproduce?
Look at the example here: [w3.org/TR/wai-aria-practices/examples/treeview/treeview-1/treeview-1a.html](https://www.w3.org/TR/wai-aria-practices/examples/treeview/treeview-1/treeview-1a.html)

#### What is the use-case or motivation for changing an existing behavior?
To make [mat-tree] more usable and accessible. Especially important when having lots of items in a tree.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
@angular: 6.1.4
@angular/material: 6.4.6
OS: Windows 10
Chrome: 69.0.3497.81
Typescript: 2.7.2

#### Is there anything else we should know?
No

Inizia con il componente mat-tree e confronta il suo comportamento attuale con TAB, SHIFT+TAB, SPACE ed ENTER con le linee guida WAI-ARIA sulle interazioni da tastiera di TreeView e con l’esempio di treeview collegato. Il lavoro è completato quando mat-tree segue il comportamento richiesto per il focus e la navigazione con le frecce, mantenendo al contempo l’interazione prevista per l’apertura e la chiusura dei nodi.

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