Fixing IsVisibleInDynamoLibraryAttribute handling for hiding nodes.

- FunctionDescriptor class has a property IsVisibleInLibrary, so that if a method with
  IsVisibleInDynamoLibraryAttribute is imported it's not added to library view, but files
  with this node can still be opened in Dynamo.
- SearchViewModel don't add function descriptors with IsVisibleInLibrary property set false.
- DynamoLoader don't add nodes with IsVisibleInDynamoLibraryAttribute to SearchViewModel so
  they are not present in library view.