Fix some bugs 2

Issue : #24 Num1 or num7 at the beginning of the timeline crashes;
	#19 Delete subtitle;
	#28 Shorter steps to accelerate

Fix :
- The text does not disappear in video window while erasing the subtitle.
- F1 after delete crashes
- Num1 or num7 at the beginning of the timeline crashes
- Bug when selecting subtile on waveform in some tricky cases
- Set of 0.2 for the video playback rate

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