Add support for Train.xml to TE2

[leezer3]

Manual rebase of https://github.com/leezer3/OpenBVE/pull/417

TE2 hasn't really been touched since the original PR, and was S520's work. With that in mind, I think this can probably be made to work with the current master, and would be a useful addition, even if some features aren't 100% complete. I'm also adding XML only features (sanders, DSD etc.) which would benefit from a visual editor.

TODO:

• New unit attribute needs adding to the main parser. (?? Combine ??)
• Fix build issues, shouldn't be anything too major.
• https://github.com/leezer3/OpenBVE/commit/71bdb397a6455777dc8ce33a3996017497c1b394 needs to be re-applied.