Bug 779565 - Treeview header combos do not work at first load

When the csv preview page is first loaded the header combo will not
work till refreshed which can be done by selecting another separator.

By adding a g_idle_add to the preview_page_prepare to rebuild the table
the combos become active.

gnucash/import-export/csv-imp/assistant-csv-trans-import.cpp

@@ -1890,7 +1890,7 @@ CsvImpTransAssist::assist_preview_page_prepare ()
         gtk_assistant_set_page_complete (csv_imp_asst, preview_page, false);
 
         /* Load the data into the treeview. */
-        preview_refresh_table ();
+        g_idle_add ((GSourceFunc)csv_imp_preview_queue_rebuild_table, this);
     }
 }