Reworked how construction is handled. Now, farms and houses are permanent objects, built on the map, and will remain there if the population flee.
Farms are built just by spending time on plains or steppe, houses require wood. Elites can build pyramids, but require wood and stone.
Forest, plains, steppe and farms now all generate food based on the season.
Optimised several issues that were worsening endgame performance.