[PICARD-519] Fix up hacky resize/restore/save of metadata box columns

Replace `metadata_box_sizes` option by QByteArray `metadatabox_header_state` and
simply rely on Qt `QHeaderView::restoreState()` and `QHeaderView::saveState()`

This change implies metadata box columns aren't auto resized when changing main window size, but instead scrollbar may appear.

Overall, it is still user-friendly enough and much more reliable.

See http://tickets.musicbrainz.org/browse/PICARD-519
Related to https://github.com/musicbrainz/picard/pull/176