Playback system rework

[toasterofbread]

This is a long-term issue for me to organise plans for some reworks to the way songs are handled in SpMp.

Needed changes:

• ~~Local songs as a distinct type of mediaitem (like with LocalPlaylist and RemotePlaylist)~~
• Rewrite download system, including download, metadata, and storage logic (#435)
• Improve auto-download (add rolling window, etc.)
• Add a way to manage active/failed downloads

[mugehan]

When do we expect to see the new version?

[toasterofbread]

@mugehan When it's ready.