Add first devtool: prev/next arrows for broken swiping in Chrome

@incuna/js Please merge, ta!
That's what I mean: requiring `Cocktail` and `Cocktail.mixin(WhateverView, devtools.mixins)` makes it that bit more complicated than I'd like for a dev to implement this. However, I'm ok with `edetail-devtools` providing a mixin (it would work well with https://github.com/incuna/edetail-project/pull/212), just not making that the sole method of implementation.