Add NativeOptions::window_builder for more customization

[twop]

Closes #2747

• this PR is my attempt to tap into winit options to style the window
• I'm building an app that behaves akin to macOS spotlight, e.g. no standard controls to minimize or close the window

here is the snippet from my app code base

 let options = eframe::NativeOptions {
        resizable: true,
        always_on_top: true,
        run_and_return: true,
        window_builder: Some(Box::new(|builder| {
            #[cfg(target_os = "macos")]
            use winit::platform::macos::WindowBuilderExtMacOS;
            // code below allows for richer customization
            #[cfg(target_os = "macos")]
            return builder
                .with_fullsize_content_view(true)
                .with_titlebar_buttons_hidden(true)
                .with_title_hidden(true)
                .with_titlebar_transparent(true);

            builder
        })),
        ..Default::default()
    };

So that makes it possible to create completely custom look and feel

Happy to rework if needed, maintaining a fork is a pain ^_^

[emilk]

Very good idea

[emilk]

Adding mut everywhere is quite ugly.

This can be fixed by using Rc<dyn Fn(…)> as the callback type, instead of just Box. That will also allow cloning.

…but we won't get to use FnOnce, which is a shame