Subtitle delay : fixed sync problem that cause audio to stop

*  WARNING, there is still a (simple) bug to fix
   At hotkeys.c:110 (function Open())
     var_AddCallback( p_intf->p_libvlc, "key-osdmessage", ActionEvent, p_intf );
     leads to libvlc_InternalDestroy:Assertion at exit

Bug reproduction step : *under linux* (will not happen under OSX)
- launch a video that has a subtitle,
- press Shift-H ("bookmark audio time"),
- wait 5 seconds
- press Shift-J ("bookmark subtitle time")
- press Shift-K ("sync bookmarks")
==> the audio is broken...

The subtitle delay ("spu-delay") was previously implemented by
  i) setting a variable spu-delay
     (var_SetTime(... , "spu-delay", ... ))
  ii) this change raised a callback that finaly lead to calling
    UpdatePtsDelay().

The reason UpdatePtsDelay() was called is that the subtitle delay
was programed the same way as the audio delay (for which it is required
to wait until the audio is actually demuxed)

*However* the situation for subtitles is quite different, as they are all in memory
(no need to wait until they are demuxed !)

The code is now more simple (and the bug is fixed !) :
- no more delay calculations inside hotkeys.c, everything is handled
  inside subtitle.c through callbacks
- inside subtitle.c :
    * adjust_subtitle_time() receives a subtitle timestamp as input and returns
      that timestamp corrected by spu-delay
    * set_current_subtitle_by_time() is called when spu-delay is changed
Added tooltips that explain the subtitle sync hotkeys.
(WARNING : I could not test it under OSX...)

This is a proposal for a tooltip in the "Track Synchronization" window
on the "subtitle track sycnhronization" label and widget.
It explains better the use of a patch I did last year that enables to
set the subtitle delay easier via hotkeys.