common.h: Add ReGBA_GetBackupFilename and ReGBA_GetSavedStateFilename.

DSTwo, GCW Zero: Create implementations of the above functions. GCW Zero: Use .sNN (.s00 through .s99) for saved state file names. Delete get_savestate_filename and get_savestate_filename_noshot, replacing them with the new functions.

memory.c: Delete references to DEFAULT_SAVE_DIR, a DSTwo port variable. Delete gamepak_filename. Replace it with CurrentGamePath, uninitialised, and IsGameLoaded, initialised to false. This makes it so the full path of the game is kept around, and initialising the first character of gamepak_filename is not needed anymore. Make load_backup, save_backup, load_state and save_state not require a path to be passed, only requiring the slot number now and using the functions above to resolve the file names per platform.