Don't accept focus for wxRadioBox itself in wxOSX

The radio box is just a static box and so can't have focus, only its
child radio buttons should have it.

This resolves the problem with getting stuck on wxRadioBox when full
keyboard access is off, as wxControlContainer code tried to give focus
to wxRadioBox because its AcceptsFocusFromKeyboard() returned true, but
none of its radio buttons could be focused without full keyboard access.

However this introduces a new problem with wxRadioBox being skipped when
full keyboard access is on, which will be fixed in the following commit.

See #18089.

src/osx/radiobox_osx.cpp

@@ -102,6 +102,9 @@ bool wxRadioBox::Create( wxWindow *parent,
     if ( !wxControl::Create( parent, id, pos, size, style, val, name ) )
         return false;
 
+    // The radio box itself never accepts focus, only its child buttons do.
+    m_container.DisableSelfFocus();
+
     // during construction we must keep this at 0, otherwise GetBestSize fails
     m_noItems = 0;
     m_noRowsOrCols = majorDim;