Added: Code and icons for tube-based transportation.

Just an additional tool that mappers may find interesting or fun. At the moment, they do not use power or any resources, and there is plenty of room to extend and improve them, but they do work.

The basic tubes will read their directions from icon_state if they aren't specifically specified, so you can just use the built-in option to generate instances from icon states to get all possible tubes.
The tube stations use directions, instead.
Merge pull request #2469 from Uristqwerty/Tubes

Added: Code and icons for tube-based transportation.