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.
Merge pull request #1331 from sharadkjaiswal/MigrationTests

Fixing IsVisibleInDynamoLibraryAttribute handling for hiding nodes.