ScrollBar refactoring

ScrollBar refactoring

1. Removed ScrollComponent and ScrollConnector
2. Further cleaned up Scollable public API
3. Refactored ScrollBar control to accept scroll position properties from customized property sources
4. Changed scroll position properties in ScrollView from Vector3 to Vector2
5. Harmonized ScrollView and ItemView to support the same new ScrollBar control

For example, to add a vertical scroll bar to ScrollView:

mScrollBarVertical = ScrollBar::New(Toolkit::ScrollBar::Vertical);
mScrollBarVertical.SetParentOrigin(ParentOrigin::TOP_RIGHT);
mScrollBarVertical.SetAnchorPoint(AnchorPoint::TOP_RIGHT);
mScrollBarVertical.SetResizePolicy(Dali::ResizePolicy::FILL_TO_PARENT, Dali::Dimension::HEIGHT);
mScrollBarVertical.SetResizePolicy(Dali::ResizePolicy::FIT_TO_CHILDREN, Dali::Dimension::WIDTH);
mScrollView.Add(mScrollBarVertical);

Change-Id: Icf61530a64585f3edc1fa33022877923414500f2