Add button for adding expression columns in Edit Index dialog

Add a button for adding new expression columns to the Edit Index dialog.

Allow the user to edit the 'name' of existing expression columns in the
Edit Index dialog.

src/EditIndexDialog.h

@@ -30,6 +30,7 @@ private slots:
     void removeFromIndex(const QModelIndex& idx = QModelIndex());
     void moveColumnUp();
     void moveColumnDown();
+    void addExpressionColumn();
 
 private:
     DBBrowserDB& pdb;