Mineclone2 compatibility

[minertestdude]

• ability to run also within Mineclone 2
• custom white UI for Mineclone2, regular (unchanged) for Minetest Game

[minertestdude]

@SmallJoker Hi! Sorry for very late response! I've gone through your messages and created another version, which already includes the changes that you implemented meanwhile, so I will open new pull request for this.

• The table inside set_string works properly on my side, example: https://imgur.com/a/XgEpYep All objects in MCL2 by default appear without flying captions over them, hence the strings are absent for mcl_core.
• As of boxart, I don't think its available. I have spent time fishing this information out and don't remember where exactly I took it - it could be that I made screenshot then used GIMP tool to get color values, because the mod uses formspec and mcl2 uses own menu (which I couldn't figure out back in day), so just found similar colors. Best