Define screen reader labels and scrollTo positions for each layer

[nateplusplus]

Problems

1. Currently, when progressing through pushin layers using a screenreader, each layer will be read simply as "Group". This could be enhanced by adding a descriptive label for each layer of content.
2. Additionally, when tabbing through layers, the browser scrolls to the inpoint of each layer. If building a composition which includes multiple layers that have the same or similar inpoints, the visual experience will not match that of a scrolling experience. It would help to have the option to provide scrollTo positions for each layer, so that the default can be overridden when needed.

Solutions

1. Add option: ariaLabel to the PushinLayer API
2. Add option: tabInpoint to the PushinLayer API - this should accept an array for different breakpoints