Set wxTAB_TRAVERSAL for wxNavigationEnabled<> windows with focusable children.

We need to have this flag under wxMSW in order for navigation events to be
generated for the window. As wxNavigationEnabled can't set this flag on
creation because its ctor is called too early, do it in overridden AddChild().

Closes #13271.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72664 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775