"Import Library..." adds nodes to CustomNodes TreeView

#### Purpose

Due to new design imported library nodes should be added to CutomNodes TreeView. To import library you should go to main menu "Library - Import Library...".

#### Modifications

For every node connected with just imported dll `DSFunctionNodeSearchElement` is created. To know that node is part of imported dll it was decided to add property `ElementType` to classes `FunctionGroup` and `DSFunctionNodeSearchElement`.

#### Additional references

[MAGN-4721](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4721).

#### Reviewers

@aosyatnik, @Benglin, please take a look.
I think these changes are not needed, ```FunctionGroup``` is only created from an external ```MyLibrary.dll``` right? So in ```SearchModel.Add``` method below, it knows that it is dealing with ```FunctionGroup```, it can directly assign ```SearchModel.ElementType.DLL``` to ```searchElement.ElementType```.