Loading freetype is now the Skin's responsibility instead of the EngineInterface's
Managing the InputBinding is now the WindowManager's responsibility instead of the Engine's
Got rid of pure virtual functions in EngineInterface that weren't being used:
    * GetScreenWidth
    * GetScreenHeight
    * quit

At this point, the only remaining EngineInterface functions are
the mouse movement/position ones, which I hope to get rid of
soon.  After that, EngineInterface will finally be dead.