expect_type for get_child_by_id and get_widget_by_id

[aaronst]

For #1237, adds an expect_type parameter to get_child_by_id and get_widget_by_id methods. I tried to mimic how this was done for query_one.

I also noticed Pylance was not happy with NodeList._get_by_id being protected so I made it public, apologies if this is an unwanted change.

• [x] Docstrings on all new or modified functions / classes
• [ ] Updated documentation
• [ ] Updated CHANGELOG.md (where appropriate)