Allow PropertyGrid DragDrop

• Introduce DragDrop operations on the PropertyGrid
• When dragging the label of a PropertyEditor, default behavior is to begin a drag operation using the current value, just like when pressing Ctrl+C
• Accordingly, dropping of suitable values should also be supported. There should be some kind of highlight effect if the operation is supported.
• BatchInfo, ContentRef, GameObjectRef and ComponentRef editors should react on the base implementation of dragdrop instead of rolling up their own drop support
• In addition to DragDrop on PropertyEditor labels, it should also be possible to do on GroupedPropertyEditor headers, but implemented in MemberwisePropertyEditors. The default implementation could perform a memberwise assignment based on the displayed values
• However, the ComponentPropertyEditor and GameObjectPropertyEditor should provide a custom CopyTo-based implementation