Added GUI for subtitle sync in the "Track synchronization" window
- 4 buttons were addd : "bookmark audio" / "bookmark subtitle" /
                        "sync subtitle" / "reset sync"
Added tooltips that explain subtitle sync
GUI for subtitle sync in the "Track synchronization" /  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.
Added subtitle speed