Actor XML loading

The building loading mechanism is completely overhauled. Now it's using the XML 'actor' files that 0AD does.

This relates to #32, buildings now show 'props' (various details) so they look a lot better. Just compare and see all the added details, it makes the buildings a lot more beautiful and interesting.

It also fixes #18, so now all buildings are available to all races / empires. I havent updated the new icons yet, that's on my todo.

Finally, it should make it a lot easier to do #56, add normal / ao / specular maps to the buildings, should we wish to do it. Because those textures are also referenced in the actor XML files.