New column page

Shows SQL for columns ALTER TABLE ... [ADD|DROP] COLUMN combines a selection
of multiple columns into a single alter table.
Show collation in list of columns.

(order of columns isn't what is should be but that should maybe be fixed
by a generic column selection and ordering mechanism that knows what the
default sort should be)

pglab/ColumnTableModel.h

@@ -2,6 +2,7 @@
 #define COLUMNTABLEMODEL_H
 
 #include "BaseTableModel.h"
+#include "PgAttribute.h"
 #include "PgClass.h"
 #include "PgIndex.h"
 #include <memory>
@@ -36,6 +37,7 @@ public:
 	int columnCount(const QModelIndex &parent) const override;
 
 	QVariant data(const QModelIndex &index, int role) const override;
+	const PgAttribute& column(int row) const;
 protected:
 	virtual Oid getType(int column) const override;
 	virtual QVariant getData(const QModelIndex &index) const override;