Subtitle delay : fixed sync problem that breaks audio under linux

    This is a follow-up to a patch I made last year
    (f7211d5efa4cd9ef392230d85fdf3470edefab39)
    When developping it I stumbled upon a prexisting bug that I could
    not correct at that time.

     Bug reproduction steps :
     ************************
     *under linux* (will not happen under OSX)
    - launch a video together with an *external* subtitle file
      (.srt file for example),
    - press Shift-H ("bookmark audio time"),
    - wait 5 seconds
    - press Shift-J ("bookmark subtitle time")
    - press Shift-K ("sync bookmarks")
    ==> the audio is broken...
    (AFAIK this bug seems not to be mentioned on trac.videolan.org)

    Correction details :
    ********************
    * Beware, there were two preexisting ways of dealing with subtitles :
	- File subtitles are handled by vlc/modules/demux/subtitle.c
        - Other subtitles are handled by vlc/modules/codec/avcodec/subtitle.c
    * A new variable was added : "sub-isfilesub".
         - equal to 1 when the subtitle is read from a file
           (for example a .srt file),
         - equal to 0 otherwise (for example dvd subtitles)
    * The subtitle delay is still stored inside "spu-delay".

    * In the case of non file subtitles, the subtitle delay
      is implemented as before: a change of "spu-delay"
      will ultimately lead to a call to UpdatePtsDelay().
    * In the case of file subtitles, the subtitle delay is implemented
      inside vlc/modules/demux/subtitles.c by the function
      adjust_subtitle_time()
      For file subtitles, the situation is different : all subtitles
      are already in memory, so that applying a delay is simpler.
    * no more delay calculations inside hotkeys.c, everything is
      handled inside vlc/modules/demux/subtitle.c through callbacks.
     This provides a better encapsulation a prepares the addition of a
     gui in the track synchronization window (patch to come later)

Note :
* this patch corrects the aforementioned bug
* however the subtitles sync hotkeys which I introduced one year ago
(Shift-H, Shift-J and Shift-K) now only work with *file subtitles*.
This should not be a problem, as I do not think this feature is of any use for DVD / DVB subtitles.
In the case of non file subtitles, they will simply not be linked to any callback.
Added GUI for subtitle sync in the "Track synchronization" window

* 4 buttons were added : "bookmark audio" / "bookmark subtitle"
                         "sync subtitle" / "reset sync"
* Those buttons are shown only for *file subtitles* (DVD and DVB are not concerned)
* Added tooltips that explain subtitle sync
GUI for subtitle sync in the "Track synchronization" under OSX : *NOT* done

As this file depends on Interface Builder 3, I could not update it
in order to add the subtitle sync feature which I added in the
Qt "Track Synchronization window".

I prefer not to mess around it, as I am not specialist enough
with interface builder...

Instead, I added comments that explain which buttons should be added and
what their behaviour should be.
Subtitle Sync : added a convenient way to adjust subtitle fps

Principle:
**********
* All of this is limited to *file* subtitles (DVD and DVB are not concerned)
* If the user needs to sync twice the delay, and if he does this at two different positions
  in the movie (separated by at least 45 seconds), a fps ratio will be calculated and proposed
  if it seems to be valid.
* The user will be asked to re-confirm the speed adjustment
* Care has been taken in order to avoid user errors:
  The subtitle fps will only be adjusted if the calculated speed ratio is close to a valid one.
  The only fps considered are { 23.976f, 24.f, 25.f, 30.f } (most common movies fps)
  A valid ratio is under the form a / b
  where a and b are members of { 23.976f, 24.f, 25.f, 30.f }
Added the possibility to save an adjusted subtitle file (.srt only)

Principle:
* Inside the "Track synchronization window" a button was added ("Save Subtitle")
* This button is only shown for *file subtitles* (not DVB, nor DVB)
* It will save the subtitles as an srt file with (the srt file name will be proposed under
  the form /subtitle/original/path/original_subtitle_file_adjusted.srt

(Also added doc for Mac OSX port inside SyncTracks.xib)