[Arp] ArpForm -> Parameters -> visible

[Mike Britton]

I may be misunderstanding, but does this mean we should code implicit
getter/setters for visible in each arpform?  Like this:

function get visible() {
    return __visible;
}

function set visible(val:Boolean) {
    __visible = val;
}

I do like the advantage of calling hide() or show() in terms of
readability, but that's just habit I guess.

Mike