Add WindowBuilder::with_parent_window (#2548)

* On macOS, add `WindowBuilderExtMacOS::with_parent_window`

* Replace Parent with Option<Id<NSWindow, Shared>>

* Add addChildWindow method on NSWindow instead

* Update with_parent_window to be unsafe fn

* Add unified `with_parent_window`

* Remove `WindowBuilderExtUnix::with_parent`

* Remove `WindowBuilderExtWindows::with_parent_window`

* Clean up CI warnings

* Update CHANGELOG.md

It's `WindowBuilderExtX11` rather than `WindowBuilderExtUnix`

* Rename parent to owner

* Make with_parent_window unsafe and update its doc

* Add another way to get window on mac

* Add more documentations

* Add match arm and panic on invalid varients

* Add Xcb arm

* Update child_window example to make it safer and work in i686

* Remove duplicate entry in CHANGELOG.md

* Propogate error instead of expect

* Replace unreachable to panic

* Add platform note to X11

Co-authored-by: Wu Yu Wei <wusyong9104@gmail.com>

src/window.rs

@@ -138,6 +138,7 @@ pub(crate) struct WindowAttributes {
     pub resize_increments: Option<Size>,
     pub content_protected: bool,
     pub window_level: WindowLevel,
+    pub parent_window: Option<RawWindowHandle>,
 }
 
 impl Default for WindowAttributes {
@@ -161,6 +162,7 @@ impl Default for WindowAttributes {
             preferred_theme: None,
             resize_increments: None,
             content_protected: false,
+            parent_window: None,
         }
     }
 }
@@ -401,6 +403,27 @@ impl WindowBuilder {
         self
     }
 
+    /// Build window with parent window.
+    ///
+    /// The default is `None`.
+    ///
+    /// ## Safety
+    ///
+    /// `parent_window` must be a valid window handle.
+    ///
+    /// ## Platform-specific
+    ///
+    /// - **Windows** : A child window has the WS_CHILD style and is confined
+    /// to the client area of its parent window. For more information, see
+    /// <https://docs.microsoft.com/en-us/windows/win32/winmsg/window-features#child-windows>
+    /// - **X11**: A child window is confined to the client area of its parent window.
+    /// - **Android / iOS / Wayland:** Unsupported.
+    #[inline]
+    pub unsafe fn with_parent_window(mut self, parent_window: Option<RawWindowHandle>) -> Self {
+        self.window.parent_window = parent_window;
+        self
+    }
+
     /// Builds the window.
     ///
     /// Possible causes of error include denied permission, incompatible system, and lack of memory.