Add island and settlement fertility

**(Do not merge yet.)**

![2013_3-fertility](https://f.cloud.github.com/assets/373440/354954/c4d09012-a0bb-11e2-82fd-f6c21f45b47a.png)
Able to:
* display resource icons

Currently assumes a 0/1 fertility system (either resource A can exist on island, or it cannot).
There are no stepdowns like "half efficiency" in the [current design] (http://wiki.unknown-horizons.org/w/Fertility) and I'm not sure they add that much gameplay value, as opposed to interface burden and code complexity (which they definitely do add).

As you may have noticed this commit only displays some random icons for settlements. **I'm looking for an implementation of the underlying concept**, even if it's rough and unpolished.
The current mapping for resources against buildings indicates that if an island is infertile w.r.t. a particular resource, the corresponding building in MAPPING should be impossible to build in settlements on that island.
How to indicate that from our build menus, message widget, buildability highlights, or a combination thereof, is up to you.

Yes, this is gathering resource<->building relations in code instead of our database. Sorry about that. Will look into wiring those in later.