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/platform_impl/macos/appkit/window.rs

@@ -213,6 +213,9 @@ extern_methods!(
 
         #[sel(sendEvent:)]
         pub unsafe fn sendEvent(&self, event: &NSEvent);
+
+        #[sel(addChildWindow:ordered:)]
+        pub unsafe fn addChildWindow(&self, child: &NSWindow, ordered: NSWindowOrderingMode);
     }
 );
 
@@ -379,3 +382,16 @@ pub enum NSWindowSharingType {
 unsafe impl Encode for NSWindowSharingType {
     const ENCODING: Encoding = NSUInteger::ENCODING;
 }
+
+#[allow(dead_code)]
+#[repr(isize)] // NSInteger
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub enum NSWindowOrderingMode {
+    NSWindowAbove = 1,
+    NSWindowBelow = -1,
+    NSWindowOut = 0,
+}
+
+unsafe impl Encode for NSWindowOrderingMode {
+    const ENCODING: Encoding = NSInteger::ENCODING;
+}