Add no_mod_screenshot.png

I prefer this to the default no_screenshot.png. It is not the best it could be - it is supposed to be a cardboard box connoting packages. Any thoughts? I think it would be better with flaps open, but any attempts I make at that fails.

Another icon idea I had was a version of the minetest logo where a side had been opened up and wires are coming out.

Old pull #1594