Merge: app.nit: intro `Text::open_in_browser` and example for the UI API

Merge: app.nit: intro `Text::open_in_browser` and example for the UI API

This PR intro a service to open a URL using the platform default browser, and an example for the UI API. The example uses most of the available controls, multiple windows and the new `open_in_browser` feature.

The changes to `core::exec` are updates for compatibility with `Text`, and some minor improvements to its doc.

Pull-Request: #2206
Reviewed-by: Jean Privat <jean@pryen.org>