Add relation type to the class structure

[rawdog71]

Classreferences right now are always interpreted as n:m relation.
It should be also possible to set classref as 1:n relation.

ToDo:

• Add relationtype to cf_attribut table - default should be n:m for compatibilty reasons. (0 = n:m ; 1 = 1:m; NULL = not set at all, but interpreted as 0 when relationref is set)
• Add the relation logic to the backend UI (Dropdown at attribut section)
• Add the logic to the backend content editor for 1:n relations (Dropdown selection to get all values of class and store it to content_integer in cf_attributcontent ->hibernate storage to new field as "ref_classname" )