Support window parenting on macOS, closes #3751

[probablykasper]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

Closes #3751
Usage:

#[command]
pub fn create_child_window(id: String, app: AppHandle) {
  let main = app.get_window("main").unwrap();

  let child = window::WindowBuilder::new(&app, id, WindowUrl::default())
    .title("Child")
    .inner_size(400.0, 300.0);

  #[cfg(target_os = "macos")]
  let child = child.parent_window(main.ns_window().unwrap());
  #[cfg(target_os = "windows")]
  let child = child.parent_window(main.hwnd().unwrap());

  child.build();
}

Implemented it in the same way as for Windows, but it would be nice if usage was the same for all platforms. Not sure what the way to implement that would be, maybe RawWindowHandle? Would also be a breaking change, unless a different name is used.

[probablykasper]

The example is failing because the Windows parent_window expects HWND but main.hwnd().unwrap() returns *mut std::ffi::c_void. Is that a bug with the Windows implementation?

[lucasfernog]

@probablykasper I think the best solution is to change parent_window to return a HWND so I pushed that, but now the child window is not visible (it works if I remove the child.parent_window() call.

[probablykasper]

Hm, no clue why that would be. Any idea if it works in tao atm?

[lucasfernog]

Parent window directly on tao is weird but at least it shows the child window. I think wry is touching the window and messing with it too. Do you know if that's the case @amrbashir