Since ANGBAND_GRAF in Hengband is not the preference file name, added an extra field...

Since ANGBAND_GRAF in Hengband is not the preference file name, added an extra field in the configuration to be what ANGBAND_GRAF will be assigned for a tile set. Largely replaced the implementation in grafmode.c with something that will work with Hengband's routines. If Hengband chooses to adopt the datafile.h/datafile.c parsing from Angband, then it would make sense to go back to Angband's version of grafmode.c.