"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 personally do not like this DLL (because it's a shortform and it's all capital letters). I'm thinking of suggesting names like ```CustomLibrary```, or ```ExternalAssembly```, or ```CustomDll```, or ```CustomAssembly```. What are your thoughts?