Add ResetScroll in Scroller API

[marc-antoine-girard]

From this post: NovaUI-Unity/Community#134 (comment)

I think adding a method in the Scroller to reset the position to its starting position would be convenient in many different contexts, but mostly when reusing the same UI for multiple pages. It's also in the case where an animation would look out of place. The solution provided in the post above works, I just think it could be either clearer in the documentation, or provided in the Scroller.

myScroller.ResetScroll();

Instead of:

myScroller.CancelScroll();
myScroller.UIBlock.AutoLayout.Offset = 0;