"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`.

#### Reviewers

@aosyatnik, @Benglin, please take a look.