"Play Date" tag was seconds since 1904 (an Apple Mac HFS+ timestamp), not a
Unix timestamp as we assumed. Seems Apple themselves realised that wasn't a
great idea (+ not a proper plist date type), and therefore provide "Play Date
UTC" as an alternative.
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.
