Fix tab navigation bug with radio boxes without enabled items.

There was a bug similar to the one in the preceding commit with radio boxes
under wxMSW too: if all radio box buttons were disabled (or hidden, although
this should be much more rare in practice, unlike the disabled case which was
deemed to be rare in r74583 commit message but turned out to actually happen),
the radio box still pretended to accept focus but didn't really do it.

Fix this by allowing to override wxWindow::CanBeFocused() and do it in
wxRadioBox to check whether we have any enabled visible items.

Also add a check for CanBeFocused() to wxControlContainer code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74586 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/containr.h

@@ -76,11 +76,12 @@ public:
     bool DoSetFocus();
 
     // returns whether we should accept focus ourselves or not
-    bool AcceptsFocus() const { return m_acceptsFocusSelf; }
+    bool AcceptsFocus() const
+        { return m_acceptsFocusSelf && m_winParent->CanBeFocused(); }
 
     // Returns whether we or one of our children accepts focus.
     bool AcceptsFocusRecursively() const
-        { return m_acceptsFocusSelf ||
+        { return AcceptsFocus() ||
             (m_acceptsFocusChildren && HasAnyChildrenAcceptingFocus()); }
 
     // We accept focus from keyboard if we accept it at all.