make get_playlist_data() generic.

Other blob types might use an analogous function as well, so introduce
blob_get_name_and_def_by_row() in blob.c, and use some preprocessor
magic to define such a function for each blob type.

Change all users of get_playlist_data() to use the new
pl_blob_get_name_and_def_by_row() and remove get_playlist_data().