Improved related object handling

*Title: Improved related object handling

Incentive:
Add support for grouping the related objects in content/edit based on their type (content class).
For instance it is a good idea to show images separately from files.

*Documentation:

New template variables:
in edit.tpl the variable $grouped_related_contentobjects is available,
it is an associative array with each group being the key and the element being
the relation list.

New INI settings:
content.ini:
 - RelationGroupSettings/Groups:
   An array with groups that should be defined, a group is defined by an identifier as a string, e.g image
 - RelationGroupSettings/DefaultGroup:
   Defines the group which all unmatched relation items are placed.

For each group defined in Groups there must be a variable called <group>ClassList where <group> is the
group identifier. The variable must be an array which should contain a list of content class identifiers
that belong to this group.


*Testing:

*SVN Revisions:
7381