Make it possible to send "scroll actions" to the Ui (without using the scrollbar).

[mitchmindtree]

Sometimes a user may want to control the scrolling in ways other than using the scrollbar, i.e:

• Dragging a widget to the edge of a container may suggest that we want the container to scroll in that direction.
• A user might want to make a click-and-drag scrolling widget.
• etc

Once implemented, we might be able to make the scrollbar use this method also.

When tackling this issue, we should also consider #602 as solving this might provide an opportunity to kill two birds with one stone.