feat scrollbar: Create getter & setter to watch scroll percentage

Improve LumX Scrollbar by adding getter and setter on the service. 
If the id of the scroll element is set, we can $watch the scrollPercent of a specific scrollbar using the getter.
I set an attr.observe ;).

About the generated ID, I don't think this feature is useful: we use an ID to watch it, if it's a generated one, we can't watch it anymore, no ?