TitleBarViewNS: Move progress text to NSTextFields from drawRect

- No longer need to redraw entire titlebar to update the text
- This speeds up receivedPlaybackProgressNotification: significantly
  - Went from approximately 6.3% to 4.3% CPU (~2% saving) during
    playback
TitlebarViewNS: only redraw progress text if it's changed

- This reduces needless screen updates (checked with Quartz Debug)
- Reduction from about 3.1% to 2.4% cpu