Chat Forever

## 0.5.2
- Performance: lowered the addon's CPU use (about 2% down to under 0.2% expected).
  - The timestamp pass now runs only when something changed (new message, scrolling, paging, resize, font change, clearing or showing the window, timestamp setting change), plus a safety tick every 0.1 s.
  - The idle fade checks mouse-over and activity 10 times a second instead of every frame. The fade animation itself still runs every frame, so it stays smooth.