Title bar seek buttons seek if held

- TitlebarSeekButtonNS:
  - Now has property called "heldAction", similar to "action", which is
    sent to the target every 0.2 seconds during a hold
  - FF/RW buttons have tag of 1/-1 respectively
- TitlebarViewNS:
  - seekButtonHeld: is the heldAction for the seek buttons
  - This sends "seekTrackByJump" notification to the music controller
    with the direction (aka the tag)
- MusicController:
  - Observe "seekTrackByJump" notification
  - Modify seek notification handler to handle absolute seeks from the
    SliderBar versus relative seeks from the buttons
    - For relative: move +/-5 seconds (except can't be less than 0)
- Current speed is +/- 5 seconds every 0.2 seconds

- Closes #46