[NUI] ScrollableBase usage improvement (#1724)

[NUI] ScrollableBase usage improvement (#1724)

Previously, user should make container for ScrollableBase to contain
children in ScrollableBase and it brought inconvenience to user.

For now, ScrollalbleBase creates its ContentContainer in Constructor so
user doesn't need to add container.

And ScrollableBase overrides Add/Remove function to add/remove child
to/from ContentContainer even if user call ScrollableBase.Add/Remove.

Plus, user can access to children of ContentContainer through
ScrollableBase.Children property and also can acess to Layout too.

Finally, if user want to change property of ContentContainer, they can
use getter of ContentContainer property.