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. 
Having to do this everywhere we instantiate a ```CustomNodeSearchElement``` is troublesome, if not error-prone. In the future anyone who creates ```CustomNodeSearchElement``` will not know they have to set the value, and for that we will have to make sure all other codes which depend on the ```ElementType``` to check for invalid types. In my opinion, we should set a default ```ElementType.CustomNode``` in the constructor, and then the part which requires it to be ```PackageElement``` should then set it accordingly. Does that make sense?