Implement .as_weak() and .upgrade() for global component instances

[bennysj]

The idea to make it easier and more convenient to pass a reference to a Slint global instance into a Rust closure / lambda.

The current implementation works the following way:

  pub fn init(blogica_api: &BLogicAAPI) {
        blogica_api.on_update_data({
            let blogica_api_weak = blogica_api.as_weak();
            |bdata| {
                let blogica_api = blogica_api_weak.upgrade().unwrap();
                // It would have been very nice if the call to .global() could avoided, as it depends on the lifetime
                // of the GlobalStrong<T> it's needed
                let blogica_api = blogica_api_weak.to_global();

                // Do stuff here with blogica_api
            }
        });
    }

Fixes #9389