Handling of activation in the generic implementation of standard renderers was
broken since r62589 which stopped calling their Activate() method. Restore it
by introducing a special WXOnActivate() replacing it (but clearly marked as
private and implementation-only) and calling it instead.
Closes#11460.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64654 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775