`windows-reactor`: Add `TreeNode::content(impl Into<View>)` method or similar

`TreeNode` only accepts a plain-text string at construction time. There's no way to set a node's content to achieve a custom layout (e.g. an image/icon next to the label).

In XAML we'd normally have something like:

```xaml

```

Start at the windows-reactor implementation of TreeNode and inspect how its plain-text content is constructed, along with the existing View APIs. Define and add a content-setting path that accepts a custom View, then verify that a tree node can display a composed layout such as an image beside a label.

Written by the indexing model from the issue text.