Solve copyright issues for 3rd-party scripts and tools

[PeterBowman]

As a corollary of #99, I run across a few issues:

• cmake/find-modules/tcbrindle_sdl2-cmake-scripts.md#licence: "The original scripts, and my changes, are released under the two-clause BSD licence." -> missing BSD 2-clause license file (compare cmake/Copyright.txt)
• cmake/find-modules/FindZBar.cmake: "Copyright (C) 2010 blah blah. All rights reserved." -> uh-oh

[PeterBowman]

+ media files (sounds, fonts, images) at share/.

Edit: see credits in README.md at root.