Don't disable wxStaticBox children at wx level when disabling it

Calling Enable() on all children from wxStaticBox::Enable() was wrong,
the actual status of the child, returned by wxWindow::IsThisEnabled(),
is not supposed to change just because its parent was disabled.

Call NotifyWindowOnEnableChange() to avoid this, while still disabling
the children visually.

include/wx/window.h

@@ -1580,6 +1580,11 @@ public:
         return false;
     }
 
+    // Recursively call our own and our children DoEnable() when the
+    // enabled/disabled status changed because a parent window had been
+    // enabled/disabled. This is only used by the library implementation.
+    void NotifyWindowOnEnableChange(bool enabled);
+
 
 protected:
     // helper for the derived class Create() methods: the first overload, with
@@ -1891,11 +1896,6 @@ protected:
     static void NotifyCaptureLost();
 
 private:
-    // recursively call our own and our children DoEnable() when the
-    // enabled/disabled status changed because a parent window had been
-    // enabled/disabled
-    void NotifyWindowOnEnableChange(bool enabled);
-
 #if wxUSE_MENUS
     // temporary event handlers used by GetPopupMenuSelectionFromUser()
     void InternalOnPopupMenu(wxCommandEvent& event);