Improve comments; add Building::make_queued() to prep for being in a queue.
Add a couple variables that the constructor missed.  Building::make_queued() prepares the building for being in a queue (for now that just means setting its construction_left value.
New enum Building_queue_status tells us what happened when we attempt to enqueue a building.  New City::expend_resource(Resource_amount).  New functions for managing City::building_queue - add a building, move it to the open list, and cancel a queued building.  Four new functions for gaining resources.
Finalize the interface for building a new building.  It includes Interface::build_building(), and three helper functions, Interface::set_building_list(), Interface::set_building_queue(), and Interface::set_building_help().