SDL no longer initializes with SDL_INIT_EVENTTHREAD.

It was causing problems on Mac OS X.
Updated Doxyfile.in with a new version of Doxygen.

It seems like the new doxygen from the git repository only capitalized
generated comments... It isn't very important, but eh!
GameSettings are now loaded and partly honored!

GameSettings are loaded at the beginning of the Game::run function, also
the Game::initializeSDL function has been changed to accept a width and
a height to initialize the window with. That means the configuration
options: width and height are the first to be honored by the engine!
Hooray!
Quick documentation fix for Game::popGameState().
Made Game's font renderer honor settings.

- Game::font_renderer_ is now just a pointer to
GameSettings::font_renderer.get(). It seems to work!
- Also documentation of that member has changed!
Added a quit function to the Game!

It exits the game as soon as possible, for use by any GameState.
Removed the PostUpdateAction system...

It wasn't really that powerful, so I scrapped it!
The Game no longer modifies the main surface.

The GameState is now in charge of clearing and flipping the screen. This
way, later on, I can implement a system allowing for a conditional render!
Game pointer -> Game reference in MenuGameState.

The MenuGameState now accepts a reference to a Game compared to a
pointer to a game. This is because the game *has* to be valid. It works
too, since the Game is guaranteed to be living during the execution of
any GameState!