Add support for sorting by multiple columns in the Browse Data tab

This allows sorting by multiple columns (as in ORDER BY Field1, Field2)
by holding the Control key while clicking on another column header in
the Browse Data tab.

See issue #1761.

src/ExtendedTableWidget.h

@@ -8,6 +8,8 @@
 #include <QStyledItemDelegate>
 #include <QSortFilterProxyModel>
 
+#include "sql/Query.h"
+
 class QMenu;
 class QMimeData;
 class FilterTableHeader;
@@ -52,6 +54,8 @@ public:
     QSet<int> selectedCols();
     int numVisibleRows();
 
+    void sortByColumns(const std::vector<sqlb::SortedColumn>& columns);
+
 public slots:
     void reloadSettings();
     void selectTableLine(int lineToSelect);