Add options for reducing the framerate and muting the window when coopdx loses focus

[tulpenkiste]

This PR adds:

• A new function to the window manager API, bool (*has_focus)(void) that checks if the window actively has focus (specifically checks for Keyboard Input on SDL)
• 2 new options in game
  • Reduce framerate on window focus lost (power saving, default: true)
  • Mute game audio on window focus lost (distraction-preventing, default: true)

The English text that I added for the new settings could probably be improved.

Let me know if theres a compilation/code issue for the DXGI and SDL1 window api backends.

Tested using SDL2 and DXGI.