Demo v3

Hello, Braincells!

Another demo, not as significant this time in terms of levels but:

• All new (licensed!) music from Tim Beek (https://timbeek.com/)
• Functional credits page
• Cleaned up menu navigation
  • You can return to the start menu at the end

That's about it for now. Next steps are some sound effects and new levels, and working on a better tile set.