Category parent always root

Patch committed. Thanks.

The 'container' field in the add/edit category form is now required, which means that a category cannot be added if there are no containers to choose from. This is the correct behaviour, because a category must have a container (even if that container is not its parent).

As for the problem of a category's parent not being choosable on the 'add category' page, there is currently no way that I can see to fix this, as some categories are not allowed to have parents, depending on the settings for their container. The category's parent(s) can always be chosen on the 'edit category' page, as the container has already been saved and defined for the category by then. For this reason, I recommend making use of the 'add child category' links on a container page.

Please let me know if the problem being referred to was not the problem that I fixed.