Add an unobtrusive version check

[nmlgc]

Now that we show the Git tag on the title screen (#11), it makes sense to add a small indicator if the current game version is out of date. Not an updater, just a small visual hint in the build info area and a menu option to open the download page in a browser, just like we do for the BGM packs.

Probably makes sense to ping GitHub rather than a self-hosted server, as it'll probably stay around longer. This would add a curl dependency, but I'm going to parse the string without a JSON library.