This patch adds an Autoproduce tag to the production queue, which means that an entity bearing this tag will autoproduce when the queue is empty.
Currently a random template from the possible entities is chosen, but it might be better to specify a list of entities to autoproduce?
Also it would be nice to let users start/stop this behaviour via a button and to be able to specify a template and amount to autoproduce.
With D2657, it would become possible to let forests grow (autoproduce trees by trees).