Scenery window

This pull request contains the full decompilation of the scenery window and the scenery set/scenery entry structures. With this commit we also have access to the build random cluster of objects tool in standard games, not only in the scenario editor. After merging the commit we should add an entry for this to the *Changes to original game* wiki page.

There are two known bugs with the current version:
* Bouncing of the scroll bar in the scroll view when all rows can be displayed in the window. Actually the bouncing/flickering was worse in the non-decompiled version, and sometimes the scroll view got stuck and you could not scroll up (this is no more).
* I tested with most of the scenery in the game, and everything is fine except for a single type of scenery (see in the image below). My first suspicion was that the tertiary color causes the rendering problem, but other three coloured scenery items render fine (see the windowed scenery in the second row next to the bad one). So maybe the error is not in the scenery window code.
![rct2_render_error](https://cloud.githubusercontent.com/assets/304384/4173026/c53c31dc-3552-11e4-9070-0dfee0151302.png)