Do not overwrite callback and optimize listing in backend

Previously you overwrite the `tl_content::child_record_callback` without remember the previous callback. This hardly reduce compatibility with other extensions. Now the default (or previous registered) callback is remembered and called instead of `parent::addCteType`.

The height limitation is also removed from the semantic html5 elements in backend listing, this make the elements more compact and improve clarity. At least line height

![semantic-html5](https://f.cloud.github.com/assets/343404/1190140/c7d229f6-242d-11e3-920a-735a62cdd30a.png)