[db] Generic fixup + align fixing up between mfi, pli and queue_item

The idea here is to make sure the fixing up of tags is done in a consistent
manner. For strings, this means stuff like trimming and empty strings -> null
are applied the same unless there are special exception rules set. It also
means that defaults are applied the same across structs, e.g. "Unknown artist"
for both mfi->artist and queue_item->artist.

The change is also necessary because we want to remove trimming from the sql
query and instead implement it ourselves.

src/library.c

@@ -133,9 +133,6 @@ library_add_media(struct media_file_info *mfi)
   if (!mfi->media_kind)
     mfi->media_kind = MEDIA_KIND_MUSIC; /* music */
 
-  unicode_fixup_mfi(mfi);
-  fixup_tags_mfi(mfi);
-
   if (mfi->id == 0)
     db_file_add(mfi);
   else