Add "rescale" tool

Issue : none

"Rescale" tool implementation :
- Recompute selected or all subtiltes timecodes, in function of 2 points
  current and new "start" timecodes.
Toolbox :
- Right clik on the index / time box to "Set from current". Set the current
value (index or time) to the box.

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