[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

picard/ui/metadatabox.py

@@ -144,7 +144,7 @@ class TagDiff(object):
 class MetadataBox(QtGui.QTableWidget):
 
     options = (
-        config.IntListOption("persist", "metadata_box_sizes", [150, 300, 300]),
+        config.Option("persist", "metadatabox_header_state", QtCore.QByteArray()),
         config.BoolOption("persist", "show_changes_first", False)
     )
 
@@ -462,43 +462,12 @@ class MetadataBox(QtGui.QTableWidget):
         font.setItalic(italic)
         item.setFont(font)
 
-    def _resize_column(self, i, size):
-        header = self.horizontalHeader()
-        nsize = max(size, header.sectionSizeHint(i))
-        header.resizeSection(i, nsize)
-
     def restore_state(self):
-        sizes = config.persist["metadata_box_sizes"]
+        state = config.persist["metadatabox_header_state"]
         header = self.horizontalHeader()
-        try:
-            for i in range(header.count() - 1):
-                self._resize_column(i, sizes[i])
-        except IndexError:
-            pass
-        self.resize_columns()
+        header.restoreState(state)
 
     def save_state(self):
-        sizes = []
         header = self.horizontalHeader()
-        for i in range(header.count()):
-            sizes.append(header.sectionSize(i))
-        config.persist["metadata_box_sizes"] = sizes
-
-    def resize_columns(self):
-        header = self.horizontalHeader()
-        width = header.length()
-        ncols = header.count()
-        visible_width = self.contentsRect().width()
-        scroll = self.verticalScrollBar()
-        if scroll.isVisible():
-            width -= scroll.width()
-            visible_width -= scroll.width()
-        if width != visible_width:
-            for i in range(ncols - 1):
-                newsize = int(round((float(visible_width) * header.sectionSize(i)) / float(width)))
-                self._resize_column(i, newsize)
-
-    def resizeEvent(self, event):
-        if abs(event.size().width() - event.oldSize().width()) > self.verticalScrollBar().width():
-            self.resize_columns()
-        super(MetadataBox, self).resizeEvent(event)
+        state = header.saveState()
+        config.persist["metadatabox_header_state"] = state