add _setlistener() function, so that slow property polling loops can be

add _setlistener() function, so that slow property polling loops can be
replaced with efficient listener callbacks. One use is the new FPS display.
This is reviewed and OK'ed by Andy, relatively trivial and separated from
the rest of Nasal, so problems are quite unlikely and confined to users of
this function.

The callback is executed whenever the property is written to -- even if
the value didn't change. The triggering Node is available via cmdarg().

Examples:  _setlistener("/sim/crashed", func {print("haha!")});
           _setlistener("/foo/bar", func { print(cmdarg().getPath() ~ " changed")})