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"

Start in the Widget Tree search entry point and trace how widget previews expose their displayed text. The issue provides no file or test names, so first locate the existing widget-name search and related tests; done means searching for preview text such as “Artisans of Southern India” returns the matching widget without relying on its name.

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