No longer use Angband 4's version of the tiles (with additional preference files...

No longer use Angband 4's version of the tiles (with additional preference files to patch up the differences with Hengband's versions).  Instead, convert Hengband's versions to PNG with the lib/xtra/graf/osx_bmp2png.py script so they're usable with the Mac front-end.  Store the results of the conversion in the repository (so the prequisites for osx_bmp2png.py aren't needed for ordinary builds; only when the tiles have to be reconverted).  Copy over the converted results during installation.