Updates and improvements for libmp4v2 version 2.0

This makes easytag compile against the newest version of libmp4v2.  I 
haven't done extensive testing, but I have tried it on a number of files 
with adding/removing/changing tags and it seems to all work fine.

Let me know if further updates/changes would be useful.

Regards,

Nick

P.S. Not sure if this list allows attachments (patch is attached).  If 
not, I'll reply to this message shortly with the actual text of the patch.

- Use new API functions
- Actually remove tags when requested instead of setting to empty string
- Report errors to log when errors occur in reading tags
---
  src/mp4_header.c |    6 +-
  src/mp4_tag.c    |  189 
+++++++++++++++++++++++++++++++++---------------------
  2 files changed, 119 insertions(+), 76 deletions(-)

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