No real changes, just put these classes in a private header. They're still not
part of the public API but at least it will be easier to reuse them inside the
library itself in the upcoming commits.
See #10258.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74336 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
