first try to allow child window on windows

examples/child_window.rs

@@ -0,0 +1,44 @@
+extern crate winit;
+use std::thread;
+use winit::os::windows::WindowBuilderExt;
+
+fn resize_callback(width: u32, height: u32) {
+    println!("Window resized to {}x{}", width, height);
+}
+
+fn main() {
+    let window = winit::WindowBuilder::new()
+        .with_title("A fantastic window!")
+        .with_window_resize_callback(resize_callback)
+        .build()
+        .unwrap();
+
+    let proxy = window.create_window_proxy();
+    thread::spawn(move || {
+        let child = winit::WindowBuilder::new()
+            .with_title("child window!")
+            .with_window_resize_callback(resize_callback)
+            .with_decorations(false)
+            .with_parent_window(proxy)
+            .build()
+            .unwrap();
+
+        for event in child.wait_events() {
+            println!("child {:?}", event);
+
+            match event {
+                winit::Event::Closed => break,
+                _ => (),
+            }
+        }
+    });
+
+    for event in window.wait_events() {
+        println!("parent {:?}", event);
+
+        match event {
+            winit::Event::Closed => break,
+            _ => (),
+        }
+    }
+}