Support zero-prefixed switch names

[freezy]

This PR adds support for switch names that are prefixed with 0. It goes up to 1000, e.g. a switch named 001 will correctly be relayed to PinMAME as 1, or 099 as 99. But it won't work for let's say 0001, in this case we'll do a normal lookup, and print an error if the ROM doesn't have a switch alias named 0001.

The implementation is somewhat hacky. We just add zero-prefixed references to the lookup tables when registering the strings and keep the rest of the code "simple". We could also have added another int-based lookup table that would work for any number. @jsm174 let me know if you prefer the latter.

See also freezy/VisualPinball.Engine#430 for the same feature for coils.

This PR also adds a new s_spawn_ball event that adds a ball on the playfield for debugging.

[jsm174]

The implementation is somewhat hacky. We just add zero-prefixed references to the lookup tables when registering the strings and keep the rest of the code "simple". We could also have added another int-based lookup table that would work for any number. @jsm174 let me know if you prefer the latter.

Looks good! I would probably prefer the later just to be cleaner and less hacky.