Allow customising ScrollContainer's scrollbar mapping

[peppy]

For use in osu!.

[bdach]

What's the use case?

[peppy]

ppy/osu#30579, sorry was writing things up.

[bdach]

My primary concern with this is that allowing this sort of override makes ScrollbarMovementExtent (which is public) sort of misleading if it is overridden the way it is in the game-side PR, because it will still appear as if the scrollbar can move the entire length of the container if reading that property, while in actuality it cannot.

Thoughts on making that protected instead or something?

[peppy]

We can try, hopefully nothing uses that in other projects.