Convert StructureCatalogue from static class to regular class

[DanRStevens]

Related:

• Add a StructureType data file #1329

The init function is basically already a constructor. We should consider using a normal class as opposed to declaring everything static. I suspect the current design was to make it convenient to access the list of available structures. We would get the same convenience from using a global variable for the class instance.

In the future, if we split some sort of game state object out of MapViewState (minus the UI stuff), we could attach the StructureCatalogue there. This may even allow for cleaner loading of saved games, where if there is an error, you could just go back to the current game. That isn't really possible with static objects, since you're overwriting global state during a load, and if there's an error during load, there's no possibility to abort and return to the previous game.