title: Write custom (user defined) MP4 field with jaudiotagger
body: <p>I'd like to write a custom/user defined field in a mp4 file with <code>jaudiotagger</code></p>
  
  <p>For FLAC files, I can do this with <code>VorbisCommentTagField</code>.</p>
  
  <p>For MP3 files using <code>ID3v24Tag</code> I can do this with a <code>ID3v24Frame</code> as <code>ID3v24Frames.FRAME_ID_USER_DEFINED_INFO</code></p>
  
  <p>Is there any way to do this in mp4? 
  Looks like there is a <code>Mp4FreeBox</code>, but I can't find a public API to use it.</p>
  
  <p>Moreover, does <code>jaudiotagger</code> expose any public API to set custom tags (key/value) in an unified way? Could not find any, and I need to treat each format independently.</p>
  
  <p>Thank you</p>