implement TC offset for slave&generator.

implement TC offset for slave&generator.

Many related changes that require atomic update, mostly
because ClockOption slots changed:

  * change offset config format to std:string (backwards compat - reads 0)
  * make Timecode offset independent from fps
  * sample_to_timecode() handle negative sample-num
  * audio-clock fix entry and edit of negative numbers
  * option editor:
    - remove old global internal offset
    - add slave & generator TC entry
    - still needs UI cleanup, tooltops, maybe sep. tab..
  * LTC & MTC slave& generator:
    - cache offset
    - subscribe to parameter changes

git-svn-id: svn://localhost/ardour2/branches/3.0@13485 d708f5d6-7413-0410-9779-e7cbd77b26cf