Unify on a single ScrollSnap

[SimonDarksideJ]

^{Issue created by Isaac Norman as Bitbucket Issue #​198 on 2018.01.03 18:18.} There seems to be more than one scroll snap that does the same thing, but each is implemented differently, with different features and faults.

Just a quick play reveals:

Scroll Snap

• doesn't support configurable speed (a couple of lines to implement)
• doesn't support pagination or pagination manager
• does work with ScrollRect Conflict Manager if you fix issue 196 by passing on events to the scroll snap script

Horizontal Scroll Snap

• doesn't work with ScrollRect conflict manager even if the right events are passed (it nearly does but jumps around)
• it's more controlling about child positioning, scroll snap lets you setup the scrollrect content normally, HSS seems to take over and size things itself, goes wrong if you've normally setup your content.