Large Scale C++ design

[Jan Wassenberg]

Cleanups inspired by book of same name (am currently reading it).

I think cleaning up our code to reduce inter-module dependencies is quite important. It promises to reduce compile time, simplify releases of parts ripped out of our codebase (e.g. a limited actor viewer), and overall make the code easier to modify (without having to wonder: "hm, what all does this break?").

First idea: sdl is dragged into 6 of 7 of our static library components - ugh! This would be mostly avoidable by just forward-declaring SDL_Event. Unfortunately it is declared as a union (bad design!!), so that is impossible. Simple solution: wrap it in a struct and forward-declare that.