Multi selection managment

Issue : none

Manage multi subtitles selection :
- Change start / end time code of several subtitles at the same time
- Shift the timecodes of several subtitles at the same time
Framerate managment :
- Recompute the subtitles timecodes when framerate change
(adjust timecode to the framerate step)

Signed-off-by: juiice <decaen.julien@gmail.com>