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.