Wayland, Windows, MacOS: Popup Implementation (#4543)

Implement proper decorationless popups by specifying the type of the child window with with_type()

With this commit, different kind of child windows can be created

-  Popups: Special windows without any decoration which can be positioned relative to the parent
-  Window: Normal window with a parent or not

The type can be specified during creation of the Window using the window_attributes and the with_type() function. As default a normal Window is used. If Popup is choosen a parent must be specified, otherwise the Popup creation fails with an Error returned by the new() function.

Related issues: #403 and #4256

winit-x11/src/window.rs

@@ -66,13 +66,26 @@ impl Window {
         event_loop: &ActiveEventLoop,
         attribs: WindowAttributes,
     ) -> Result<Self, RequestError> {
-        let window = Arc::new(UnownedWindow::new(event_loop, attribs)?);
-        event_loop.windows.borrow_mut().insert(window.id(), Arc::downgrade(&window));
-        Ok(Window(window))
+        use winit_core::window::WindowType;
+        match attribs.window_type() {
+            WindowType::Window => {
+                let window = Arc::new(UnownedWindow::new(event_loop, attribs)?);
+                event_loop.windows.borrow_mut().insert(window.id(), Arc::downgrade(&window));
+                Ok(Window(window))
+            },
+            WindowType::Popup => Err(RequestError::NotSupported(NotSupportedError::new(
+                "Popups are not implemented for X11",
+            ))),
+            _ => Err(RequestError::NotSupported(NotSupportedError::new("Unsupported window type"))),
+        }
     }
 }
 
 impl CoreWindow for Window {
+    fn window_type(&self) -> winit_core::window::WindowType {
+        winit_core::window::WindowType::Window
+    }
+
     fn id(&self) -> WindowId {
         self.0.id()
     }