Provide possibility to switch from connected mode to free translation mode (and vice versa)
In some cases it might be desirable to change the translation mode for a page.
- changing from connected to free. This should be possible (currently it is possible by setting l18n_parent to 0 for each record, but this is tedious).
- changing from mixed to free
- (changing from free to connected might also be possible but as this is probably more tricky and must search for the translation parent by following l10n_source I think it is ok, to omit this option.)