Bug fix - video full scale on height

Issue : none

When the video is full scale on height, the text edition field position
is wrong. Bad computation. This patch fix it.

Signed-off-by: juiice <decaen.julien@gmail.com>
Bug fix - re-init waveform

Issue : none

Re-init the waveform when a new video is loaded.

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

Issue : none

- Replace "F5" by "Delete" key to delete the current subtitle
- "PageUp" key : select the previous subtitle
- "PageDown" key : select the next subtitle
- "[NUM]1" or "F11" key : move current position 1 second after
- "[NUM]2" or "F12" key : move current position 1 second before

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