Fix copy in internal buffer

There was an omission of the last_row update in
968b8d55f0

That had an effect of converting all copied selections to one column.

src/ExtendedTableWidget.cpp

@@ -250,6 +250,7 @@ void ExtendedTableWidget::copy(const bool withHeaders)
                 lst.clear();
             }
             lst << indices.at(i).data(Qt::EditRole).toByteArray();
+            last_row = indices.at(i).row();
         }
         m_buffer.push_back(lst);