Most players/tools can cope with MP3 files which have some bytes of junk
after the ID3v2 tag just fine, e.g. ffmpeg shows just
```
[mp3 @ 0x559e1f4cbd80] Skipping 1670 bytes of junk at 1165.
```
and most players can play the file just fine.
This change makes the tag editor also more resilient, allowing it to skip
a certain amount of junk bytes before a known container is detected. It
will also skip a certain amount of junk MPEG audio frames.
For now let's just ignore these elements explicitely until they are
actually supported. This way the warnings are at least more specific and
there will be no inconsistency when updating the track language.
"DATE" is an official field and "YEAR" only an inofficial one but present
in some files. In consistency with MediaInfo and VLC player it is treated
like "DATE" here.
It just looks nicer to have e.g. "2017" instead of "2017-01-01T00:00:00".
The ID3v2.4.0 standard and the Matroska standard explicitely allow this
format as well.
1. Convert TYER and related fields of old ID3v2 versions to the new TDRC
field and only expose that via the generic accessors.
2. When writing an old ID3v2 tag, convert TDRC back to the old fields.
3. One can still manually unset the via 1. auto-populated TDRC to disable 2.
and write the old fields directly. So the automatic handling does not
reduce the flexibility of the library.
4. Deprecate 'Year'; it is replaced by the already existing 'RecordDate'
which is now supposed to be used everywhere where 'Year' was used before
5. Introduce 'ReleaseDate' to support this field which is supported in
ID3v2.4.0 and Matroska via the generic accessors.
6. Use ISO format when converting tag values of the type DateTime to/from
string. This is closer to what's used in ID3v2 tags internally. (The
library still allows the old format as fallback when parsing for
compatibility.)
* Close or flush streams explicitely so writing is not
deferred
* to catch errors in the right place
* to avoid suppressing errors completely when writing
would be deferred to the destructor invocation
* Improve comments
Martchus