[chusan] Added rudimentary favorites support to aqua-viewer

aqua relevant merge request: aqua!16 (merged)

Recently picked up on Chunithm after a trip to Japan and I've been wanting to rebuild my favorites list but unfortunately the option was missing from aqua-viewer, after a bit of looking around the code I managed to add a simple version of it. I've only spent a few hours on this on a completely alien codebase in a language I never use, so if I did something super stupid please tell me and I'll try to fix it. When I say rudimentary, I mean:

• It'll not stop you from pressing the button if there are > 50 songs in your favorites (which is the limit from what I've read)
• There is no list to easily see all the songs you have favorited, so you have to find them again in the song list/database
• Additionally, since I'm only interested in New+, the code will only work with Chusan

For the function itself, it is present on any music detail page and looks like this (ignore the missing picture):