*Title: Improve content class structure

Incentive:
Make it possible to read out from a content class if it were meant
to be used as a container or not. This allows the template system
to only perform something (e.g. controls for creating subitem) when
it is set or not.

*Documentation:
A new field called 'is_container' is added to ezcontentclass table.
It can contain either 0 (false) or 1 (true).

The content class export system was updated to handle this new field
and is set as an attribute on the root XML node. The attribute is
named 'is-container' and contains either 'true' or 'false'.
e.g.
<content-class is-container="true">
<content-class is-container="false">

Upgrading from previous eZ Publish releases (3.4 or lower) will require a script
to be run or manual fixation by an administrator.

Note: The admin interface requires this to be set correctly for folders and
other container-like classes. If not it is impossible to create subitems.

*Testing:

*SVN Revisions: