Add ability to search by the text in a widget preview

Currently we can search in the Widget Tree by the Widget name. One of the suggestions that came up in the walkthrough was to be able to search by the text in the widget (which is displayed as part of the widget previews).

E.g. for the widget below, instead of searching for `Text` (which might find hundreds of widgets), search for "Artisans of Southern India"

从 Widget Tree 的搜索入口开始，追踪 widget 预览如何提供其显示文本。该 issue 未提供文件名或测试名称，因此先定位现有的 widget 名称搜索及相关测试；完成的标准是，搜索“Artisans of Southern India”之类的预览文本时，能够返回匹配的 widget，而不依赖其名称。

由索引模型根据 Issue 内容生成。