Define `visible` for the `Stage` class

[adroitwhiz]

There are a couple places in the codebase where we have an object that is a Sprite or a Stage, and we need to check if it's visible. Because visible is only defined for Sprite, this is awkward and requires us to do "visible" in target ? target.visible : true.

Instead, we could define a visible getter for Stage that always returns true, and just omit the setter. If JS allows it, we may be able to just define the getter on SpriteBase and only define the setter on Sprite.