DbAddDel: Replace use of TreeView::remove_all_columns().

* glom/utils_ui.[h|cc]:
  Add a treeview_delete_all_columns() method to explicitly
  delete the columns instead of just removing them and sometimes
  implicitly deleting them with Gtk::TreeView::remove_all_columns().
* glom/mode_data/db_adddel/db_adddel.[h|cc]:
* glom/utility_widgets/adddel/adddel.cc:
* glom/import_csv/dialog_import_csv.cc: Use it.

This seems to fix a crash when switching between Data and Find mode,
though that might just have been due to not resetting the pointer
to the button column.