Allow configurable speed factor when ItemView is being swiped.

[problem] No way to configure scroll speed factor separately while panning or flicking.
          Therefore for some layouts you can't get a value to make both panning and
          flicking work.
[cause] Only one scroll speed factor for both panning and flicking.
[solution] New API for configurable speed factor while flicking.

Change-Id: Ia58a4346db0923410fd61005efb38f4d1ca4a843
Signed-off-by: Adeel Kazmi <adeel.kazmi@samsung.com>