Prevent duplicating wxPG property items on the list of items for deferred deletion/removal.

It is necessary to prevent duplicating items on the list of items to be deleted/removed later on (in wxPropertyGrid::OnIdle) to avoid crashes when it would be attempted to delete/remove already deleted/removed item.

See #16222.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76888 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775