Trim release dates to year

This makes it consistent with how dates are handled in expr.c

Most tag decoders trim the dates anyway (see e.g. fix_date() in id3.c and ffmpeg_read_comments() in ffmpeg.c). For decoders that don't (e.g. flac.c), the sorted library view looks broken when full dates are intermingled with year-only dates:

```text
 2. En lintinbluitin faran...             04:46 Falkenbach                            2013-11-01 Asa                                   Metal - Folk
 3. Return to Ultima Thule                03:25 Falkenbach                            2013-11-01 Asa                                   Metal - Folk
 4. I svertar sunna luihtint              05:59 Falkenbach                            2013-11-01 Asa                                   Metal - Folk
 1. Introduction                          01:42 Fall                                  2009 Black Autumnal Void                   Metal - Black
 2. Trance in Pain                        07:33 Fall                                  2009 Black Autumnal Void                   Metal - Black
 3. Black Autumnal Void                   06:14 Fall                                  2009 Black Autumnal Void                   Metal - Black
```