The mp4 tagger.

This commit adds support for editing meta tags of mp4 files. We
employ the mp4v2 library to do the work, so fairly little code has
to be added to the aac audio format handler.

We could deactivate tag editing for mp4 files if libmp4v2 was not
detected, but since subsequent commits for the aac audio format
handler will be based on the mp4v2 library as well, we keep it simple
and deactivate aac support completely in this case.