One key difference to the Rust way is what `slint::Window` means. In
Rust that holds the `WindowInner` and `slint::Window` is only exposed as
`&slint::Window`. This is possible because the component owns the
`Rc<dyn PlatformWindow>`, which has a function to return the
`&slint::Window`.
In C++ `slint::Window` is also exposed as `slint::Window&` in the
`window()` getter, but there's no way to get a reference to a C++
wrapper for the Rust `&slint::Window` that the `PlatformWindow` trait
returns. Therefore in C++ `slint::Window` wraps `Rc<dyn
PlatformWindow>`.
This was a regression following the move to the edition 2021.
The CallbackUserData was not capture in the lamda, its member were moved.
So the destructor was called right after it was set.
We must make sure we capture the whole CallbackUserData, so put the callback
inside of it. This also reduce a bit of code duplication at the same time.
Test the callback invokation with statefull lambda
