rct2: Add support for extracting assets from demo packages

Adds the two demo sources from openrct2.org as mentioned on d-devel-games.

Note that the .exe is a rar archive, and when it is extracted an explicit list of files to extract is constructed that apparently excludes any marked as "doc", so there's a warning about manual.pdf?demo not being found.

The second commit is a cleanup changing "?gog" -> "?ttp" for versions of files that come from a Triple Thrill Pack install (CD, GOG, or now the demos).