Update some comments
test_python_module: Unref the python object.
test_document_export_po: Remove the test file.

* glom/libglom/utils.[h|cc]:
  Add delete_file() and catch exceptions in delete_directory().
* tests/translations_po/test_document_export_po.cc:
  Delete the file that the test creates.
test_selfhosting_new_then_alter_table: Uncomment an extra test.

* tests/test_selfhosting_new_then_alter_table.cc: Check for use
  of quotes in the table name now that the bug in libgda seems to
  be fixed.
main.cc: Do not call gtk_init() now that we can use newer gtkmm.
AppWindow: Remove some unused code.