Add Support for Downloading All Songs in an Album or Playlist

[MarcZierle]

First off, I want to thank all contributors of Koel. The website/server and app both look amazing and work really well. Now to the pull request:

These changes address issue #63 for downloading all songs of an album (or playlist). Basically, I copied the SongCacheIcon to SongListCacheIcon which expects a list of songs instead of a single song. I modified the code slightly to update its download/downloading state based on all songs in the list. Furthermore, the download() method downloads up to three songs in parallel. To inform each song about its downloading state, which can now change from outside (i.e. the album or playlist) I also added a new StreamController to broadcast when a download has been initiated.

If there are any further changes or tweaks you'd like me to make, please don't hesitate to let me know.

[phanan]

Hey, thanks for the PR! Is there a reason other lists like Artist and Favorites are not supported?
Also, I hesitate to add download support for lists like these because of the potential problem(s) with big lists that can contain thousands of items.

[sylphrena0]

I'd just say that an button to download all songs in a library would also be great (as long as it works with thousands of items, my library is 2,500+ songs).