Scalable Dropdowns update

In our initial meeting on Wednesday (IRC log), we outlined some steps and a fairly aggressive timeline to get ourselves in a place where changes to wp_dropdown_users() (#19867) and wp_dropdown_pages() (#9864) are suitable for patch review.

To that end, we’ve opened a handful of issues on GitHub for work and discussion, so that we can track completion. These tasks include evaluating various accessibility areas (touch, mobile, keyboard, screen reader), creating a JS wrapper, UI skinning, and page hierarchy representation. Please check them out and participate as you are able and willing. Let’s keep those issues task-oriented, however – any other questions or comments can go here.

#4-1, #dropdowns, #plugins