Provide stand-in assets for all files loaded from Civ3

[WildWeazel]

Again this will be an epic, but just capturing for reference. @esbudylin has compiled a list of files that are loaded from Civ3 as of Carthage. We will need to provide stand-ins (permanent or placeholders) or otherwise define our own data for all of these in order to run standalone, plus any others that are referenced later.

In summary, most of these comprise:

• A full terrain set
• Units for Worker, Warrior, Chariot, Catapult, and Galley
• Domestic advisor
• Advisor and general popups
• Buttons
• Civ color palettes
• Basic UI sounds
• Main menu music

https://gist.github.com/esbudylin/91e76979c63ceff20c6c2f9f57f3779a