Characters number management

Issue : none

Characters number maximum per line :
- Display characters number per line (in red when number exceed the max)
- Wrap text to an other line when characters number exceed the max
Characters number per second :
- Display the characters number per second (in red when > max)
- Adapt the duration of the subtitle in function of characters number
- User can Switch on/off auto duration (switch off when timecode
  changed manualy
Other :
- Set black background to the videoplayer
- Fix bug on F3 insertion

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