Import the TS temperate tileset.

This introduces the basic framework for a TS -> OpenRA tileset converter, and does an initial port of the temperate tileset.  This is still quite limited by the constraints of our tileset format, but I plan on fixing this by incrementally modifying our tileset and re-running the converter in stages through future PRs.

The tileset can be tested using my [ingame map editor](https://github.com/pchote/OpenRA/tree/ingame-map-editor) branch.