*Title: Improved eZObjectRelationList datatype

*Incentive: 
The existing objectrelation datatype has several limitations:
- that's always a single relation (no multiple choise)
- the layout is not what we wanted (list with check boxes and list with
  radio buttons)
- you can't easily retrieve all the objects that are related to a specific
  object (reverse related)

*Documentation:

Features:
Some new features have been created:
- that is able to handle a one to one or one to many relationship (it allows multiple select),
- that let you define the target objects list based on their parent node.
- that provide two new interfaces two manage the relation: a list with radio button (single selection) 
  and a list with check boxes (multiple selection)
- you can select the potential related objects as 1) the children of a node
- 2) all the objects from a given class. 
- With the template based selection - modes, we have defined a new view (objectrelation.tpl) that you
  can override in case you'd like use

You can choose one of 'Selection methods' when editing/creating of a class with eZObjectRelationList

Selection methods:
 - Browse - (by default)
 - Dropdawon list
 - List with radio buttons
 - List with check boxes
 - Multiple selection list
 - Template based, multi
 - Template based, single

If you choose 'Object Class' and 'Default location' 
you can create new objects as childs of 'Default location' and as objects of class 'Object Class'.