Added Great Work details to proposed deal section

Per requests from @Aristos and joerd9 on reddit (https://www.reddit.com/r/civ/):
- Available gold per turn is now displayed on the gpt icon
- Great works contain additional information on their buttons/tooltips for easier trading.

Fixed bug in early game where deal screen would throw an error and cease to populate inventories if you occupied an opponent's city, and they only had one city remaining.