Handling vinyl sides in tracknumber tags.

[simoncn]

In this scenario, there are no invalid tags.

1) adds TRACKNUM tags (for A/B values only) in addition to TRACKNUMBER tags

3) replaces all TRACKNUMBER tags with TKN tags, so now we have a mixture of TKN and TRACKNUM tags. These tags don't have any value checking, so they can't be invalid.

4) creates TRACKNUMBER tags with a TRACKNUM value if present (the A/B case only) or a TKN value (all other cases). All these values are integers, so nothing is invalid.