Ensure that widgets can be garbage collected on each platform

[samschott]

This WIP PR starts by adding tests for the garbage collection of widgets. Known issues causing test failures are:

• Circular references between winforms and Python. Will be fixed by #2066.

Other test failures will need to be investigated.

PR Checklist:

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