Add keystrokes for stepping -/+5 and -/+10 frames at once

This is something that exists in the current video coding software at Babylab/LPNC, and that the users find very handy.

We could use:

• Left: backward one frame
• Ctrl+Left: backward 5 frames
• Shift+Left: backward 10 frames
• Right: forward one frame
• Ctrl+Right: forward 5 frames
• Shift+Right: forward 10 frames

Also, we could add buttons to the video manipulation QHBoxLayout for -10, -5, +5, and +10 frame steps.