Ensure that all code snippets, tutorials and demos are working

code snipperts - https://github.com/enthought/traitsui/tree/master/docs/source/tutorials/code_snippets
tutorials - https://github.com/enthought/traitsui/tree/master/examples
demos - https://github.com/enthought/traitsui/tree/master/traitsui/examples/demo

The focus of this issue is to ensure that all external-facing code examples in traitsui are working as expected (on Qt and Wx toolkit backends if both are supported). Specifically,

- [ ] the code runs without any errors. Any errors should be fixed.
- [ ] the code don't raise any warnings or messages on the commandline. Any warnings/messages need to be debugged and fixed or documented in issues.
- [ ] the code itself is well documented e.g. docstrings in the module, classes and functions/methods.

Note that moving/refactoring the code or adding new code is out of scope for this issue