Introduce visual and scrollable container

Usually visual and scrollable are exactly the same, scroll calculation and display are both done using the same container. So here is the idea : 

The scrollbar could be calculated for a certain container that overflow but it could be display on a different container or just in a part of the scrollable container. 

Let's see with an example from jsfiddle : [http://jsfiddle.net/dB7zd/3/](http://jsfiddle.net/dB7zd/3/)
In this example we have a long text and a button both part of the scrollable container e.g. the div with the overflow:hidden. Usually the scrollbar rail would be all over this container and had the same height. But here, while we continue to scroll the entire container, the scroll bar only appears over the text and not over the button. Our visual container is smaller than our scrollable container.

For now it's a first shot and thus is very minimal (only handle Y-axis, element at the bottom, no test over argument etc...) but this pull-request is more about the idea than the actual pull-request.