"Play next" for artists and albums

[Whateverest]

Hi, thanks for the great App!

The "Play Next" function currently only works for individual tracks.
I was used to having this feature for whole albums and artist as well, so I implemented it in my fork.
I tested it on my personal device and with an emulator - it seems to work fine.

I think the "most interesting" part of the code (meaning the one most likely to have bugs), is the new method: PlaybackService.enqueueAsNextTracks.

The implementation borrows from both the enqueueTrackAsNext and the enqueueTracks methods.
Songs are added after the current playing song, but if no song is currently playing, the newly enqueued album won't start playing (like it would when enqueuing tracks via enqueueTrackAsNext).

Are you interested in integrating this into odyssey?
If not, feel free to close this PR :)