"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.
Hi @Benglin, unfortunately method `SearchModel.Add(IEnumerable<FunctionGroup>)` is executed for regular libraries too. For example I am debugging now and see this collection is populated of approximately 600 members. So we need somehow specify in `FunctionGroup` that it is created for imported library.