Added "visibleFrom" to Modifier

How to manipulate visibility?

* Write custom .render() function in a view or component
* Use a RenderController - does complicated stuff to track transitions in and out
* Use a Scrollview - does complicated stuff to only render what is visible

What about simply showing/hiding things? Use cases

* Popups or notifications (without animations)
* Pages in a simple website
* Basically every use case for render controller without animations.

I've added a `visibleFrom` method and a `_visible` flag. RenderSpec will be `null` if visible is `false`.