Extend and adjust Scrollbar themeing

- add "active" state
- add "opacity" property
- make inactive scrollbars transparent
- make scrollbar background transparent
- change scrollbar width to 8px

Change-Id: I7fb274e0c174aeb3a5e19edf42b3985a10d1c896
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>