[NUI.Components] DropDownButton to use Layout scrolling (#1160) (#1161)

[NUI.Components] DropDownButton to use Layout scrolling (#1160) (#1161)

Squashed:
Now uses NUI Layouting for the drop downbox
DropDown API fixes Delete/Select
LayoutScroller immediate positioning
Applied LayoutScoller updates
LayoutScroller now a CustomLayout with OnMeasure and OnLayout implemented
Velocity of flick not throttled hence can be fast.
Updates after Style refactor

Features:
Persistant selection Icon/Checkbox
Highlight for touched selection
Scrolling list
Styled using ControlStyle system.

Change-Id: I656bc978331df5f38e1885635ba50b6b5e178976