Built-in media player (beta feature)

The only option to play file is currently to "Open in Player" which is useful but has some drawbacks:

• Available media players must be satisfactory for use with Picard, see PICARD-1487;
• Picard has no control over the player, thus it cannot be stopped when user quits Picard and when user selects another file within Picard.

For this last reason mainly, it would be very convenient to have a built-in player, along the existing "Open in Player" feature.