Add support for updating Opus metadata.

This requires a taglib version with Opus support, but such a
version has not yet been released, even though support has been in
git almost a year. This version is scheduled as version 1.9

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
Update the language when saving metadata.

Otherwise it won't get saved to the file.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
Fix a memory leak in ParseVorbisComments().

Also, don't abort parsing the rest of the comments if we encounter
an empty one.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
Show Save Metadata button when editing comments.

QTextEdit doesn't have a textEdited() signal, but textChanged()
works well enough if we disable it while we're changing the text
ourselves.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>