Bugfix: grouping taxonomies get reset in overviewss.

When saving content and taxonomy via the overview (i.e. anywhere except via the post function in `editcontent`), the grouping gets removed. Took me quite a while to see what was going on:

Default (same thing you see in frontend):
```
/chapters/meta => Meta Chapter
```
Specifically via `Content->setFromPost`:
```
0 => meta#0
```