Add auto-barter to trade UI and global toggle to trade agreements

[sizzlins]

• Adds an 'Auto Barter' feature to the trade window, calculating required values and automatically selecting merchant/fortress goods to match an internal margin factor.
• Adds a global 'Select Globally' toggle hotkey to the diplomacy requests UI to select/deselect requests from all available tabs.

[sizzlins]

Auto Barter, automatically selects depot goods to match chosen merchants good with a * 1.25 profit margin for the merchant. Expensive means selecting the most valuable item first to trade with. Cheap means choosing the cheapest items first to trade with.

[sizzlins]

Select Globally, selects all item requests for next years caravan diplomacy.