Add frame rate managment

Issue : none

- The timeline is now stepped in function of the frame rate

Add some controls
- "[NUM]-" or "F7" key : decrease video playback rate
- "[NUM]+" or "F8" key : increase video playback rate
- "[NUM]7" or "F9" key : move current position 1 frame before
- "[NUM]8" or "F10" key : move current position 1 frame after
- "[NUM]4" or "ctrl + P" key : switch play / pause video playback

Signed-off-by: juiice <decaen.julien@gmail.com>
Change "font" and "position" managment

Issue : none

Before, to retrieve "font" and "position" infos, data was computed from
the text edit zone coordonate.
Now "font" and "position" infos are retrieved directly from the subtitle
database if a subtitle is indexed for the current time.
If not, the default infos are retrieve.

When a "font" or "position" parameter are changed from tool boxes:
if there are no subtitle indexed for the current time, the parameter become
the default parameter.

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