Farming update

Updates seeds into comestibles, adds a minimum and maximum time for them to grow and randomizes growing, and cleans up some code using furniture.load_id in grow_plants in map.cpp.

Edit: I do still need to readd the old seeds for backwards compatibility and redo the item spawns, I realized.
You can't count on the submap being processed all the time, it needs to also work if you leave for a long time and then come back.