More carts

Implements #3421.

* Welding cart: 1/24 chance of being found in a garage.
* Food cart, Ice cream cart: Both can be found in the park that's not a basketball court.
* Wheelbarrows: Found in and around hardware stores.
* Luggage cart: Found in the entry area of hotels.
* Swivel chairs: Found in offices (small ones, and towers).

Also adds a small storage battery (1/100th the storage of a full-size battery, basically for powering kitchen units and welding rigs without giving you tons of power for free), and vehicle-mounted minifridges (which don't do anything... yet. They're just there for flavor at the moment).

I kind of took my best guess at spawning the vehicles and used the option to have them not destroy walls or create wreckage, so any suggested adjustments to the add_vehicle calls are welcome.