Add wxNavigationEnabled<> helper for implementing TAB navigation.

Add wxNavigationEnabled<> helper for implementing TAB navigation.

Provide a clean, public and documented way to implement proper TAB navigation
for subwindows of a composite control: instead of using various ugly and never
documented WX_XXX_CONTROL_CONTAINER macros it is now enough to simply inherit
from wxNavigationEnabled<BaseClass> to do it.

No real changes in the code as the new class is not used anywhere yet.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68363 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775