Add initial popup implementation

src/Avalonia.Windowing/PopupImpl.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Collections.Generic;
+using Avalonia.Controls;
+using Avalonia.Input;
+using Avalonia.Input.Raw;
+using Avalonia.Platform;
+using Avalonia.Rendering;
+using Avalonia.Windowing.Bindings;
+
+namespace Avalonia.Windowing
+{
+    public class PopupImpl : WindowImpl, IPopupImpl 
+    {
+        public PopupImpl(IWindowWrapper wrapper) : base(wrapper)
+        {
+        }
+    }
+}

src/Avalonia.Windowing/WIndowingPlatform.cs

@@ -36,7 +36,7 @@ namespace Avalonia.Windowing
     {
         internal static WindowingPlatform Instance { get; private set; }
         private readonly EventsLoop _eventsLoop;
-        private readonly Dictionary<IntPtr, Window> _windows;
+        private readonly Dictionary<IntPtr, WindowImpl> _windows;
 
         public WindowingPlatform()
         {
@@ -44,7 +44,7 @@ namespace Avalonia.Windowing
             _eventsLoop.MouseEvent += _eventsLoop_MouseEvent;
             _eventsLoop.Awakened += _eventsLoop_Awakened;
             _eventsLoop.Resized += _eventsLoop_Resized;
-            _windows = new Dictionary<IntPtr, Window>();
+            _windows = new Dictionary<IntPtr, WindowImpl>();
         }
 
         void _eventsLoop_Resized(IntPtr windowId, ResizeEvent resizeEvent)
@@ -100,13 +100,17 @@ namespace Avalonia.Windowing
 
         public IPopupImpl CreatePopup()
         {
-            throw new NotImplementedException();
+            var windowWrapper = new GlWindowWrapper(_eventsLoop);
+            var window = new PopupImpl(windowWrapper);
+            _windows.Add(windowWrapper.Id, window);
+
+            return window;
         }
 
         public IWindowImpl CreateWindow() 
         {
             var windowWrapper = new GlWindowWrapper(_eventsLoop);
-            var window = new Window(windowWrapper);
+            var window = new WindowImpl(windowWrapper);
             _windows.Add(windowWrapper.Id, window);
 
             return window;

src/Avalonia.Windowing/WindowImpl.cs

@@ -10,12 +10,12 @@ using Avalonia.Windowing.Bindings;
 
 namespace Avalonia.Windowing
 {
-    public class Window : IWindowImpl
+    public class WindowImpl : IWindowImpl
     {
         IWindowWrapper _windowWrapper;
         private LogicalPosition _lastPosition;
 
-        public Window(IWindowWrapper wrapper)
+        public WindowImpl(IWindowWrapper wrapper)
         {
             _windowWrapper = wrapper;
         }