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.

src/common/statboxcmn.cpp

@@ -105,11 +105,15 @@ bool wxStaticBoxBase::Enable(bool enable)
               ++i )
         {
             if ( *i != m_labelWin )
-                (*i)->Enable(enable);
+                (*i)->NotifyWindowOnEnableChange(enable);
         }
 
         m_isEnabled = enable;
 
+        // Notice that we don't call DoEnable() on the box itself: under MSW it
+        // doesn't actually change anything and under GTK this would disable
+        // the label window.
+
         return true;
     }
 #endif // wxHAS_WINDOW_LABEL_IN_STATIC_BOX