Add Cocoa clipboard handler to replace Carbon/SDL2 handlers for Mac OS.

[AliceLR]

This adds a Cocoa clipboard handler to directly access the clipboard instead of relying on the Carbon (discontinued) or SDL2 (expects a UTF-8 encoded input) handlers. Note this hasn't really been tested on older Macs yet so I'm not sure if replacing the Carbon handler entirely with it is a good idea.

Mac OS Roman encoding is used for the clipboard data because it seems to be the only easily supported text encoding that can have all 256 codepoints cleanly converted to/from UTF-8. ISO Latin-1 and Windows 1292, for example, have issues with various codepoints in the 128-159 range.

Fixes this issue.

• Don't use array literals (LLVM 3.1).
• Don't use autoreleasepool (LLVM 3.0).
• More testing.
• iMac testing: Snow Leopard.
• iMac testing: pre-10.6 implementation.
• iMac testing: PPC toolchain/DMG.
• Actual testing of that DMG on a real PowerPC Mac..
• Xcode project?
• Check into linking warnings about stub files and library files being out of sync (probably a configuration error on my end). Exporting SDKROOT in ~/.profile fixed this issue.