Rework validation methods

- Introduce validate_many methods
  so inputs could be validated together and not
  one by one
- Drop value_or_notification in favor of single
  value_or_none (which doesn't notify by default when
  FailedValidationError occurs - because it is already
  visible in the TUI)
- validate_with_notification -> validate_passed with similar logic as
  value_or_none, but it should be used where
  additional validation is required