Fix Content widget misbehavior

Starting from QT 5.10.1 Content widget is messed up and its settings are
lost while restarting.
Setting StretchLastSection property to false seems to solve the issue.
(Closes #8439).

src/gui/torrentcontenttreeview.cpp

@@ -43,6 +43,7 @@ TorrentContentTreeView::TorrentContentTreeView(QWidget *parent)
     QTableView unused;
     unused.setVerticalHeader(header());
     header()->setParent(this);
+    header()->setStretchLastSection(false);
     unused.setVerticalHeader(new QHeaderView(Qt::Horizontal));
 }