Added music, achievements, bug (lol) fixes, and some other things

Remember how I said when the game first came out that I’d update it with music soon™? Yeah, I spent way longer doing that than I really wanted to.

Some other changes have also been made:

• Background music, courtesy of @RyanAndBreezy
• Achievements, fifteen in total: most of them aren’t super hard to unlock, but some of us find this kind of thing to be a bit of fun
• Enhanced the look of the UI a bit. Most importantly, the level select screen shows a preview of the level in the background
• Added a translation system, although the only language currently available is English because I’m sadly monolingual and all of my polyglottal associates are tied down at the moment
• General bug fixes; fortunately there didn’t seem to be anything major, aside from a few graphical issues
• Some balance changes

I’d like to do at least one more update for this featuring actual translations and maybe some more content later, but this’ll do for now. In the deep future I might make a “premium” version with way more content in it or something.