Branch ??: Share null widgets

```
Currently lots of widgets use an empty spacer (Spacer((0, 0))) as a null
widget. Each of them typically creates his own instance as a class member.

Create a shared global enough.gui.null_widget.null_widget and have all
those that use spacer use it.

```

Original issue reported on code.google.com by `Simon.Yo...@gmail.com` on 9 Nov 2007 at 6:16
