Mark scrollbar as "active" if you can use it for scrolling

Scrollbar can be used for scrolling if scrollable widget is focused
(scrolling by keyboard or tab) or hovered by the mouse (scrolling by
mouse wheel or scrollbar itself). Mark scrollbar as "active" in this
case.
This change is a preparation for hiding unactive scrollbars by themeing.

- create ScrollBarsActivator
- mark active scrollbars with "active" state
- conver ScrollIntoView to plain JS object
- small adjustments to Scrollable.js and ScrollIntoView.js

Change-Id: I4326940611725fdf9c04e6c4fd34ab57c1dcfed6
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>