Custom games and gameplay improvements

• New game mode in which the victory condition and the initial resources of the countries can be customized. It's also possible to play as none and watch simulations with custom data. There is no time restriction in this mode so saving is manual and loading is instant without replay.
• AIs are now less reckless and are more likely to annex nearby territories.
• Nuclear weapon and geographic random events effects have been increased and now AIs may retaliate with nukes if they are bombed or invaded.
• Urban planning and economic stimulus now also grant a portion of economy and population respectively, and their effect drains resources from the rest of the world.
• Combat with militias now damages the surrounding territory.
• Demographic and economic growth will now slow down progressively to prevent resources from ramping up too high in very long games.
• Minimum number of players for multiplayer is now 2 until custom multiplayer games are available.

These changes break saved games so the previous build is available in the beta branches in case someone wants to finish a saved game.