Add support to "Play next"

[sergiopvilar]

We need to implement a queue playlist so the user can add more episodes to queue.

Acceptance criteria:

• The user should be able to add episodes to queue by using the contextual menu, clicking with the right button of the mouse over an episode
• The first episode in the queue should play right after the current episode ends
• When the triggers to play an episode, the queue should be maintained
• A button in the toolbar should be introduced to show a pop-over with the queue (see image below)

Technical suggestions Would be great have a class to manage the queue, could be binded to PlayerController (renderer).

[sergiopvilar]

The commit 5b26bed35b9f8a119c96e24e241bf62a17ec40bb introduces the Amplitude.js player that supports queue of songs.