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-android/src/event_loop.rs

@@ -769,9 +769,15 @@ pub struct Window {
 impl Window {
     pub(crate) fn new(
         el: &ActiveEventLoop,
-        _window_attrs: window::WindowAttributes,
+        window_attrs: window::WindowAttributes,
     ) -> Result<Self, RequestError> {
-        // FIXME this ignores requested window attributes
+        if window_attrs.window_type() == window::WindowType::Popup {
+            return Err(RequestError::NotSupported(NotSupportedError::new(
+                "Popups are not implemented for Android",
+            )));
+        }
+
+        // FIXME this ignores the rest of the requested window attributes
 
         Ok(Self {
             app: el.app.clone(),
@@ -825,6 +831,10 @@ impl rwh_06::HasWindowHandle for Window {
 }
 
 impl CoreWindow for Window {
+    fn window_type(&self) -> window::WindowType {
+        window::WindowType::Window
+    }
+
     fn id(&self) -> WindowId {
         GLOBAL_WINDOW
     }