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. 
I wouldn't like to do it in constructor. Because in near future tasks `CustomNodeSearchElement` can be marked not only as `CustomNode` but also as `PackageElement`. I would prefer in all copy methods specify `ElementType`. I will do it right now.