Fix plain text copy to print selected columns

The case being fixed is when non contiguous columns were selected, e.g.
column 3 and 7

client/xtableview.h

@@ -167,8 +167,9 @@ private:
             }
             text.append("\t"); // column header for row number/title
             for (int i = start; i < end; i++)
-                text.append(model()->headerData(i, Qt::Horizontal)
-                                        .toString()+"\t");;
+                if (indexes.contains(model()->index(indexes.first().row(), i)))
+                    text.append(model()->headerData(i, Qt::Horizontal)
+                                            .toString()+"\t");;
             text.append("\n");
         }