Add window icon on non-Windows platform

I noticed that the icon is set only on Windows so I fixed that using SDL. As SDL2_image fails to load the ico (certainly because it doesn't know wich image in it to load) I just used a png. If the loading fails, it will fallback to the default icon. I let you choose wether to keep this on non-Windows platform or not but IMO, the lesser platform-specific, the better.