Ng-zorro-antd: [0.7] 建议 nz-tree 开放 NzTreeService 相应接口

[cipchk]

What problem does this feature solve?

在很多数场景下，我们希望把 Tree 当成表单的一部分，所以如何获取已选中全部项成了一个刚需，而目前 NzTreeService 有足够多的方法可以帮助我完成这个事件，但我非常难得到它们。

What does the proposed API look like?

有两个方向的建议。

NzTreeComponent 提供更多日常API

例如，使用以下方法可以获得所选项（或其他）。

<nz-tree #tree></nz-tree>

@ViewChild('tree') treeComp: NzTreeComponent;
getAll() {
    this.treeComp.getSelectedNodeList();
    // 或
    this.treeComp.tree.getSelectedNodeList(); // 其中 `tree` （或其他名称）表示 `NzTreeService` 实例
}

nodes 双向绑定

用户对 Tree 的触发直接响应数据源变动。

[ng-zorro-bot]

It will be better to write your issue/comment in English, so more people can understand you.
And this means that more people can help you or benefit from your issue/comment.

[ming311]

how to use Bi-directional binding, can you tell us in detail?

[simplejason]

will support ngModel instead of nzTreeData @cipchk @ming311

[wxingheng]

I would like to guide nz-tree on how to support bidirectional binding?Or do I want to update this component?
@simplejason @cipchk @ming311

[simplejason]

@wxingheng #1277

[ming311]

it's better to give an example of using nz-tree's two-way data binding and this component needs to add some method to operate it.
@wxingheng

[libgdxlover]

好想骂人，你们有用过这个tree写过业务吗？连获取选中的节点都做不到，难道就展示个树在哪玩？图个好看？？

[vthinkxie]

@libgdxlover blocked.

[lock[bot]]

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