Add `Slint.open-url(string)` function

[tronical]

Feature Description

We want to have a function called Slint.open-url() in the Slint language. That will make it easy to write StyledText { clicked(url) => { Slint.open-url(url); }}

The implementation should delegate url opening to the platform backend; qt uses built-in API, winit/android are the crates that depend then on webbrowser.

Product Impact

No response