Interface for adding production items to a building's queue.
Comments.  Recipe now has two ints, units_per_day and days_per_unit, that track how fast the recipe produces the result.  Only one should be used.
Add build rates to the existing recipes.
Commenting & whitespace.
In Interface::building_status(), don't allow the user to jump to the production list on non-producing buildings.  Move all code for selecting a new item for the production queue to a new function, Interface::pick_recipe().  Allow the user to remove items from a production queue.