text: Title: Add type information to `TestSuite` structure
  Body: The `TestSuite` structure would benefit from more type hints that would allow IDEs to provide better completion support. That would help with writing code using the visitor API and listener API v3, especially if #4569 and #4568 are implemented.
  
  For most parts adding type info ought to be pretty easy and at least those parts should preferably be done already in RF 6.1. It is possible that `TestSuite` code using our `@setter` is too dynamic to editors to understand and just adding type info to isn't enough. If that's the case, we need to consider changing such code so that it uses the standard `@property.setter` instead. That may be too big a task for RF 6.1, though.