Added some graphics options, regarding window size and higher resolution for the 3d rendering. 

• Basic gives you a small window
• Pixel art provides a larger window, and rendering is pixelated (this is what has been until now the only rendering option)
• High detail provides the same larger window, but the 3d renders in high resolution now (probably more GPU intensive than the previous two options)

I have some trouble regarding how the UI gets rendered in Godot, so that prevents a more elegant solution. This will have to do for the moment.