Add wxItemContainer::DetachClientObject() and use it in wxRearrangeList.

Add wxItemContainer::DetachClientObject() and use it in wxRearrangeList.

Add a method to detach the item from an item control without deleting it and
use it in wxRearrangeList to correctly swap object client data without
deleting the pointers in the process.

Closes #12201.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65040 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775