VideoPlayerViewController: Apply custom seek duration
This merge request brings changes to the custom seek duration that is now applied:
- When the user uses the swipe gestures to seek forward or backward.
- When the user interacts with the backward / forward buttons in the player controls.
- When the user presses the left / right arrow keys with an external keyboard to seek backward or forward.
Closes #122 (closed)
A cosmetic patch is also added in this merge request, applying Swift syntax to the seekBy property declaration to respect the consistency and remove warnings.