VISION object is singleton, accessible via vision from GRAPHIC
This object is responsible of "graphic mode" initialisation and
graphic events management.
This class give access to the events' loop (see also start)
and to the display size.
When you create this new loop, existing windows will be
insensitive and jobs will be suspended. Then you create
your new window and all it's widgets and finally call start.
The state is "restored" when you break this loop.
NOTE: loops may be reused (restared) when you need to
reuse the same modal window.