Merge pull request #1702 from AvaloniaUI/fixes/clipboard-settext-win32

fix win32 clipboard settext async.

src/Windows/Avalonia.Win32/ClipboardImpl.cs

@@ -57,6 +57,9 @@ namespace Avalonia.Win32
             }
 
             await OpenClipboard();
+
+            UnmanagedMethods.EmptyClipboard();
+
             try
             {
                 var hGlobal = Marshal.StringToHGlobalUni(text);