WIP: Simplify Badging API for first attempt

[fallaciousreasoning]

Changes

• Remove string/grapheme badges.
  • Not supported on iOS/Android
  • Windows API only gives us 16x16 pixels to play with, so they'll be hard to see.
• Move Service Worker out of initial attempt. API needs more thought
  • Service worker scope isn't necessarily the same as PWA scope. Which PWA do we badge if a service worker has multiple PWAs installed?
  • Some platforms (Windows 7) only support badging for an open app window.

[mgiuca]

Also could you update the description:

• Link to Remove strings from the first version of this API #17 which I just filed for this issue.
• Rephrase, avoiding "what is actually being implemented". We aren't thinking of it as a "spec follows implementation" (since Chrome is just one possible implementation). In fact we could easily have implemented the character part; this change is being driven by feedback we've received from the intent-to-implement saying we should start with a simpler API surface. So rather, say we are simplifying the API for the first attempt.

[mgiuca]

Approve, please make one minor change requested below.

[mgiuca]

Merged, thanks for doing this!