[brd] Add support for more than 2 layers

[abetusk]

At least four layers, more if we can. In theory this shouldn't be so bad. There needs to be quite a few places though that need to check for collision. Through hole parts need to check each layer. Of course all other collisions need to restrict to the proper layer.

The auxiliary scripts that generate gerbers and such also need to be updated.

estimate: 5
priority: medium

Boards are getting cheaper and the need to keep footprints small is going to eventually win out. Combine that with all major free tools really only offer 2 layers while proprietary tools tend to charge for more than 2 layers, this might be a nice addition to meowcad.