R1:2ad521b96d39

Categorize UIExamples a little bit

Summary:
Ref M1476. I'm planning to add some PHUIX examples, but sort out the existing examples a little first. I added some categories:

- Catalogs: these are what I look at most often (emoji, icons, colors).
- Single use: elements with only one use (badges, feed stories, hovercards, setup issues).
- Technical: examples which are really just "test this thing in the browser" (avatars, gestures, notifications, remarkup).
- Other: evrything else, mostly general…