Fixes #17190: SerializationException on ListViewItemCount (#17195)
* Fix issue #17190: SerializationException on ListViewItemCount
ListViewGroup doesn't (always) have a serialized member "ListViewItemCount".
This patch accounts for that and also adds support for the "ItemsCount"
property that upstream [1] seems to use.
Also items should be initialized regardless of the count, otherwise we get a NPE.
[1] https://github.com/dotnet/winforms/blob/master/src/System.Windows.Forms/src/System/Windows/Forms/ListViewGroup.cs#L144