Fix tab navigation into radio boxes in wxMSW.

Fix tab navigation into radio boxes in wxMSW.

Radio boxes refused to take focus from keyboard as their
wxControlContainer::AcceptsFocusFromKeyboard() always returned false because
the base wxStaticBox class disabled setting the focus to the control itself
and wxRadioBox doesn't have any children at wx level in wxMSW.

Fix this by reenabling "self focus" in wxRadioBox to make it possible to
accept focus from keyboard. This is not ideal as it doesn't take into account
e.g. radio boxes without any items or with all items disabled or hidden, but
this should be rare and would require virtualizing all children access at
wxControlContainer level, i.e. would be quite non-trivial so don't do this for
now as this, at least, fixes the navigation in common/normal case.

Also remove the unnecessary AcceptsFocus() override from wxRadioBox as this is
now done at wxControlContainer level.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74583 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775