Light-emitting and toggleable furniture

* Two new furniture properties: "on" and "off". These work similarly to the "open" and "close" properties.
* The ^ command can be used to switch furniture on and off.
* Furniture objects can now emit light in a circle or cone; the furn_t and item classes now share the light_emission struct.
* Furniture now has a signed "power" value which can be set in .json definitions. It doesn't do anything yet, but will be used in the future for devices that consume or produce electricity.

This is the first step toward electrical systems outside of cars, though it doesn't actually add in any electrical things yet. Look good so far?