Added the following shortcuts (and made them configurable):
 - Shift-H: sets a timestamp bookmark on the audio
 - Shift-J: sets a timestamp bookmark on the subtitle
 - Shift-K: corrects the delay between both bookmarks
 - Command-Shift-K: resets the delay

See http://forum.videolan.org/viewtopic.php?f=7&t=109641 for an
illustration of a possible use
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>