Allow for SlidesContainer in edetail-lib@v1.0.1

@incuna/js I'm merging this and releasing straight away so we can get this into edetail-project
The idea was to provide a very easy way to add these helper arrows: it should be able to be added to pre-existing files in the base project, so I configured it to go in `app/app.js`. Otherwise the dev must find the right place to put it and know how to mix it in. I aimed for speed and ease of use, so it can be used in projects both before and after `edetail@1.0.0`