CustomNodeSearchElement during Dynamo loading process should be marked as "ElementType.CustomNode"

Super small but important change.

During Dynamo loading process CustomNodeSearchElement should have value `ElementType.CustomNode` (not `Regular` value for now). [MAGN-5089](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5089).

@Benglin, please, take a look. 
Can we do this in the ```CustomNodeSearchElement``` constructor? I also see the constructor being used in ```NodeSearchElement Copy()``` but we do not set the ```ElementType = ElementType.CustomNode```, is that expected?