Dont freeze if someone implements window close cancellation.

src/Avalonia.Controls/WindowCollection.cs

@@ -96,7 +96,7 @@ namespace Avalonia
         {
             while (_windows.Count > 0)
             {
-                _windows[0].Close();
+                _windows[0].Close(true);
             }
         }
 
@@ -131,4 +131,4 @@ namespace Avalonia
             }
         }
     }
-}
+}