Background for DMG file (OS X disk image)

[krzys_h]

Now I see here some old files from old @OdyX NSIS installer. Is that installer still used somewhere? I tested recent GOLD-0.1.6 installer and there was no such images like colobot_nsis.bmp and colobot_nsis_left.bmp. So I guess we can clean up here and add new, official installer images with their sources for all OS platforms.

I think that installer is still buildable, just not used by anyone since we're using my NSIS script for official builds