This closes #29. It's also progress towards #56.

The External Player is implemented the same way as VLC or Mpv. It required a couple changes to how the Player classes work, but it can still be used the same. Thus, --player external is needed to activate it. There's detailed information about how it can be set-up in the README.md.

It uses Network Service Discovery (zeroconf) so that clients can connect to the Vidify server, and then sends the video information continuously via JSON packets over TCP. The protocol is described inside vidify.player.external.

For now, only the server sends packets to the clients. In the future, the clients will need to send special packets to the server with necessary information about the device and such. That part is implemented but currently has no use.

Changes to the Player interface:

• Now they can either take a YouTube link, or a direct link to the video, depending on what player.DIRECT_URL's value is. YouTubeDLWorker now returns a dict rather than the direct URL str. The URLs can be extracted with get_direct_url or get_youtube_url, later on.
• player.position no longer has a setter. It's been renamed to seek, which supports both relative & absolute positions. This was necessary because some players like this one don't have access to their own position, and because some players like mpv have different methods for relative/absolute seeking, which may be more efficient.
• The players are now correctly deleted when the app is closed. vidify.gui.window now has a closeEvent method that will de-initialize it. This is needed to close the server sockets and unregister the NSD service.
• Players have undefined behavior when the video's properties are modified before a video plays.

This branch also includes a couple fixes for bugs I found while modifying the repo. One of them fixes a bug where if no songs are playing on the Spotify Web API, it would crash (because metadata.item is null). Another one fixes the Player custom attributes, which were terribly implemented and didn't work.