Added prolog facts for perception calls when querying for currently visi...

...ble objects

Use it like this:
```lisp
(crs:prolog `(currently-visible-objects ?template ?obj))
```
The `?template` parameter is an object designator holding the object description prototype to pass to the perception subplan. In case you don't want to specify a template, omit the parameter:
```lisp
(crs:prolog `(currently-visible-objects ?obj))
```
The objects found are then bound to the parameter `?obj` for each result.