[NUI] Scrollbar : Separate the concept of viewport size and view size. (#1632)

[NUI] Scrollbar : Separate the concept of viewport size and view size. (#1632)

* Previously, viewSize represents viewport size, but in some cases they are not the same value.
* View size : Implement OnRelayout of Scrollbar to respond to response to size changes.
* Viewport size : Receive by user when initialize.

Signed-off-by: Jiyun Yang <ji.yang@samsung.com>