AtspiNode API: changes to consider

[spectranaut]

When adding documentation, I noticed the following few things:

• [ ] AtspiNode::getRelations
  • rename to getRelationSet to match API
• [ ] AtspiNode::getTargetForRelationAtIndex
  • pass string instead of index in that array? :/
  • also don't throw if target is null, we should only throw if we get an error code
• [ ] AtspiNode getChildren
  • this should wrap atspi_accessible_get_children, because we might want to test that API specifically
• [ ] thoughts about "toString": #213
  • should ToString never throw, as a helpful utility function?
  • should ToString print ALL information, or just some?
• [ ] also: https://github.com/Igalia/acacia/issues/154
• [ ] actions interface get_name is deprecated: https://docs.gtk.org/atspi2/method.Action.get_name.html