Add Next/Previous buttons to magnetic scroll

[SimonDarksideJ]

^{Issue created by @SimonDarksideJ as Bitbucket Issue #​352 on 2020.10.22 00:19.}
Proposed by @margulis.d

Enhance the new Magnetic infinite scroll to include Next/Previous buttons.

*Admin note, likely look to extract Next/Previous button logic from Scroll snaps and create generic capability for ScrollSnaps