Forward declare DC in window.h rather than including dc.h
Forward declare Arena and MappableFile in fs.h rather than including files.
Forward declare Chunk* in shader_db.h rather than including chunky.h
Forward declare Chunk* in model.h rather than including chunky.h
Remove unnecessary include from essence_panel.cpp
Forward declare Hashable in shader_db.h rather than including hash.h
Remove lingering dependency on wxWidgets

Note that this dependency was conveniently pulling in a zLib implementation, which is now done explicitly.