Merge demo and tutorial examples

Referring to `traitsui/examples/demo` and `traitsui/examples/tutorials`

From this https://github.com/enthought/traitsui/pull/808#discussion_r424535947:

> There are two entry points because before this year there were two completely different sources: this demo.py and a separate tutor.py that included a completely parallel and differently architected demo application.
>
> This is now a merging of the two codebases so that it can handle both sets of examples. At some point the traits demo and tutor examples should be merged (with likely a lot of clean-up!)

This is likely an issue that requires a stream of PRs, not just one.
Opened for tracking purposes.