Allow using windows that can't be focused with wxNavigationEnabled<>.

Allow using windows that can't be focused with wxNavigationEnabled<>.

The code in wxNavigationEnabled<> assumed that the window itself could be
focused if it didn't have any focusable children because it was originally
extracted from wxPanel that can, indeed, be focused. This is however not the
case for all windows, notably not for wxStaticBox which now derives from
wxNavigationEnabled as well but can never be focused itself.

Add wxControlContainer::DisableSelfFocus() and call it from wxStaticBox to
support this situation. This required splitting m_acceptsFocus flag into
m_acceptsFocusSelf and m_acceptsFocusChildren and updating various methods
using them.

See #13271.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72663 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775