Working out main game loop

- Add SDL_INIT_TIMER_AUDIO_VIDEO_EVENTS to sdl_h.ads
- Clean up with and use statements
- Rename C to ReturnCode for clarity
- Add some SEH to see how it works
- Add adventure_loop.adb and adventure_loops.ads
  - First steps into OOP with Ada
  - Encapsulate main game loop and state within Adventure_Loop class