Post audit cleanup

[freakboy3742]

Builds on #2075.

Final cleanups after the completion of the widget audit:

• Removes the dummy first argument from uses of event handlers
• Removes the @not_implemented decorators, as the implementation tests are no longer needed.
• Removes the unittest helpers, as the test suite is now all pytest.
• Replaces pytest-freezegun with pytest-freezer, due to the lack of maintenance on the former.

All platform should now be running without generating any warnings during testing.

Fixes #2171

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Repeating the comment here so it isn't forgotten - we need to run all the example apps to make sure they're using current APIs once #2075 is finalised.

[freakboy3742]

I've just run and updated all the examples. This picked up a couple of places where testbed tests weren't as robust as they could have been (they were just checking that the handler was invoked, not the arguments on the handler invocation), so those tests have been updated.