Add support for exchanging gold with AIs. Trying to solve multiplayer desync issue.

Keep open border in deal to allow selling continuously to other AIs. Allow opting out notifications.

* Fix a bug when there's a "surprise" diplomacy session during offer fetch. Like Aztec's complaint on player's extra luxury resources.
* Add hotkey "D" for opening the popup (configurable in settings). Due to some limitations, hotkey for closing is not supported. Please use ESC to close the popup instead.

Fixed a missed variable rename which caused loading failure.

1. Support changing gold per turn to gold.
2. Support selling open border agreement.
3. Support bulk adding strategic resources and diplomatic favors.
4. Fix hot seat issues.

Fixed resource amount for AI to sell strategic resources in notification.

Fix a bug where offer fetch stuck when AI is not responding. Correct AI's needed amount for late game strategic resources.

Increase file load order.