[Data Object Tree] Add className property

[markus-moser]

For the D&D to relation attributes we need the className system field from the GDI in the tree endpoint.

[markus-moser]

Related to #566

[markus-moser]

An important aspect here is that we need to respect the user permissions!

[lukmzig]

@markus-moser not sure what do you need here ...the class name property is existing in the tree endpoint already. Please elaborate what should be changed, what do you need regarding the permissions

[markus-moser]

@lukmzig Sorry, you are right, it's already there at the endpoints. So no todos left - need to check it on frontend side where it gets lost ;) Thanks for checking!