feat(material/tree): revamp Tree data model, a11y and documentation

Iterate on Tree component to improve its data model, accessibility and documentation.

Introduce levelAcessor and childrenAccessor as an alternative to TreeController. Given a data node, childrenAccessor determines the children of that node. Given a data node, levelAccessor determines level of the node. One of levelAccessor or childrenAccessor must be specified, not both.

Implement keyboard navigation in Tree component to align with [WAI ARIA Tree View Pattern](https://www.w3.org/WAI/ARIA/apg/patterns/treeview/).

Make changes directly to Cdk tree API, which also apply to Mat tree APIs. See “Breaking Change” and “Deprecated” sections below for specifics on adopting changes.

Accessibility improvements
 * Implement keyboard navigation for CdkTree and MatTree
 * Implement keyboard usability for CdkTreeNodeToggle.
 * Improve ARIA semantics for CdkTree, CdkTreeNode, Tree and TreeNode components
 * Make accessibility fixes to tree and cdk-tree examples
 * Add accessibility instructions to documentation

Documentation updates
 * Add API and usage examples for TreeKeyManager
 * Update @angular/cdk/tree and @angular/material/tree to be more consistent
 * Update examples to use levelAccessor and childrenAccessor
 * Add example for (activation) on MatTreeNode and CdkTreeNode

BREAKING CHANGE: Tree component uses a levelAcessor or childrenAccessor as alternative to a controller.
 - CdkTree requires either a levelAccessor or childrenAccessor as Input. Cannot specify both.
 - CdkTree optionally takes a expansionKey as Input. expansionKey is a function that returns key of a given node. CdkTree uses key to determine expanded state.

BREAKING CHANGE: By default, tree nodes cannot be expanded and collapsed. Use isExpandable function to specify nodes that can be expanded and collapsed.
 - Add isExpandable function to NestedTreeControlOptions to determine if argument tree node can be expanded or collapsed.
 - Add isExpanded Input to CdkTreeNode to specify the expanded state. Has no effect if node is not expandable.

BREAKING CHANGE: CdkTree exposes methods and Output to manage expanded state.
 - Add isExpanded method to CdkTree
 - Add toggle, expand and collapse methods to CdkTree
 - Add  toggleDescendants, expandDescendants, and collapseDescendants methods to CdkTree
 - Add expandAll and collapseAll methods to CdkTree
 - Add expandedChange Output to CdkTreeNode

BREAKING CHANGE: CdkTree exposes method and output to manage active state.
 - Add activate method to CdkTreeNode
 - Add activation Output to CdkTreeNode

BREAKING CHANGE: Use isDisabled to set the disabled state of tree nodes.
 - Add isDisabled Input to CdkTreeNode. By default, nodes are not disabled.

BREAKING CHANGE: MatTree and CdkTree components responds to keyboard navigation.
 - CdkTree and MatTree respond to arrow keys, page up, page down, etc.; Keyboard commands align with [WAI ARIA Tree View Pattern](https://www.w3.org/WAI/ARIA/apg/patterns/treeview/).
 - Add TreeKeyManager class
 - CdkTree consumes TreeKeyManager
 - Can no longer set the tabindex on MatTreeNode. MatTreeNode ignores the passed value of tabindex. MatTreeNode always has a tabindex attribute of “-1”.

BREAKING CHANGE: CdkTreeNodeToggle directive and component respond to keyboard navigation.
 - CdkTreeNodeToggle toggles expanded state on Enter or Space keypress

DEPRECATED: Tree controllers deprecated. Use one of levelAccessor or childrenAccessor instead. To be removed in a future version.
 * BaseTreeControl deprecated
 * TreeControl deprecated
 * FlatTreeControl deprecated
 * NestedTreeControl deprecated
 * treeControl Input on CdkTree deprecated in favor of using one of levelAccessor childrenAccessor Inputs.

DEPRECATED: Setting tabindex of tree nodes deprecated.
 * `@Input tabIndex` on `MatTreeNode` deprecated. MatTreeNode ignores Input tabIndex and manages its own focus behavior.
 * `@Input defaultTabIndex` on `MatTreeNode` deprecated. MatTreeNode ignores Input defaultTabIndex and manages its own focus behavior.
 * `@Input tabIndex` on `MatNestedTreeNode` deprecated. MatTreeNode ignores Input defaultTabIndex and manages its own focus behavior.

DEPRECATED: disabled Input renamed to isDiabled. disabled to be deleted in a future version.
 * disabled Input deprecated and alias to isDisabled
 * disabled Input deprecated and alias to isDiabled

src/cdk/a11y/a11y.md

@@ -27,7 +27,7 @@ Navigation through options can be made to wrap via the `withWrap` method
 this.keyManager = new FocusKeyManager(...).withWrap();
 ```
 
-#### Types of key managers
+#### Types of list key managers
 
 There are two varieties of `ListKeyManager`, `FocusKeyManager` and `ActiveDescendantKeyManager`.
 
@@ -55,6 +55,64 @@ interface Highlightable extends ListKeyManagerOption {
 
 Each item must also have an ID bound to the listbox's or menu's `aria-activedescendant`.
 
+### TreeKeyManager
+
+`TreeKeyManager` manages the active option in a tree view. This is intended to be used with
+components that correspond to a `role="tree"` pattern.
+
+#### Basic usage
+
+Any component that uses a `TreeKeyManager` will generally do three things:
+* Create a `@ViewChildren` query for the tree items being managed.
+* Initialize the `TreeKeyManager`, passing in the options.
+* Forward keyboard events from the managed component to the `TreeKeyManager` via `onKeydown`.
+
+Each tree item should implement the `TreeKeyManagerItem` interface:
+```ts
+interface TreeKeyManagerItem {
+  /** Whether the item is disabled. */
+  isDisabled?: (() => boolean) | boolean;
+
+  /** The user-facing label for this item. */
+  getLabel?(): string;
+
+  /** Perform the main action (i.e. selection) for this item. */
+  activate(): void;
+
+  /** Retrieves the parent for this item. This is `null` if there is no parent. */
+  getParent(): TreeKeyManagerItem | null;
+
+  /** Retrieves the children for this item. */
+  getChildren(): TreeKeyManagerItem[] | Observable<TreeKeyManagerItem[]>;
+
+  /** Determines if the item is currently expanded. */
+  isExpanded: (() => boolean) | boolean;
+
+  /** Collapses the item, hiding its children. */
+  collapse(): void;
+
+  /** Expands the item, showing its children. */
+  expand(): void;
+
+  /**
+   * Focuses the item. This should provide some indication to the user that this item is focused.
+   */
+  focus(): void;
+}
+```
+
+#### Focus management
+
+The `TreeKeyManager` will handle focusing the appropriate item on keyboard interactions. However,
+the component should call `onInitialFocus` when the component is focused for the first time (i.e.
+when there is no active item).
+
+`tabindex` should also be set by the component when the active item changes. This can be listened to
+via the `change` property on the `TreeKeyManager`. In particular, the tree should only have a
+`tabindex` set if there is no active item, and should not have a `tabindex` set if there is an
+active item. Only the HTML node corresponding to the active item should have a `tabindex` set to
+`0`, with all other items set to `-1`.
+
 
 ### FocusTrap