[Class editor] Improvements

[vin0401]

There still some improvements we need to take care of:

• After creating an class the general setting are not always preselected.
• Before the submit of the form we have to validate for field-definition names.
  • They either have to be unique
  • or need be unique under a certain scope (e.g. blocks).
• Dragged items for custom layouts should be opened after dropping them from the class sidebar.
• Unique identifier for new classes is not always recomputed.
• Icons in general settings are currently not savable.
• Items can't be dragged on base node.
• check general/perspective permissions