player: Introduce the Playback Popover

This widget allows the user to browse through the app without losing track of the current playlist (what's playing next).

It is based on this wireframes:

This feature has been worked on Outreachy by Jordana Luft, but that branch differs entirely from the current state of the code base. Its been two years.

Fixes #14

---

(Screenshot)

This is the first interaction towards the full implementation of the wireframes.

Despite lacking a few of the wireframe's features, it is a step on that direction which is fully functional (could be merged).

TODO:

• Embed the shuffle/repeat buttons in the popover (Could be a #newcomers bug)
• Mark the previous played song(s) (Could be a #newcomers bug)
• Limit the length of the playlist (Could be a #newcomers bug)
• Have different views for: playing an album, playing a playlist, playing the whole library. (hard)