add the "visible" radio_property to all objects.

[John W. Eaton]

On 15-Dec-2007, Shai Ayal wrote:

| I attach a patch to add the visible property to all objects. The
| property is of type radio_property with the allowed values of
| "{on}|off"
| 
| Shai
| 
| 
| 2007-12-15  Shai Ayal  <shaiay at users.sourceforge.net>
| 
|         * graphics.h.in (all object classes) Add the visible property.
|         * graphics.cc (all object classes) Ditto.

If the visible property appears in all graphics objects, then I think
it should probably be added to the base_properties class and inherited
by all the others.

jwe