win32 - fix activate method, now same as wpf.

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -1110,6 +1110,9 @@ namespace Avalonia.Win32.Interop
         [DllImport("user32.dll", SetLastError = true)]
         public static extern IntPtr SetActiveWindow(IntPtr hWnd);
 
+        [DllImport("user32.dll", SetLastError = true)]
+        public static extern bool SetForegroundWindow(IntPtr hWnd);
+
         [DllImport("user32.dll")]
         public static extern IntPtr SetCapture(IntPtr hWnd);
 

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -507,7 +507,7 @@ namespace Avalonia.Win32
 
         public void Activate()
         {
-            SetActiveWindow(_hwnd);
+            SetForegroundWindow(_hwnd);
         }
 
         public IPopupImpl CreatePopup() => Win32Platform.UseOverlayPopups ? null : new PopupImpl(this);