Cascading windows

[freakboy3742]

Derived from #2244.

Ensure that each new window gets a new position, cascaded from the previous window with a default position.

Also splits the window testbed tests into 2 modules, making it easier to run the window size/position tests without also running the longer dialog tests.

Fixes #2023.

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

[mhsmith]

Also splits the window testbed tests into 2 modules, making it easier to run the window size/position tests without also running the longer dialog tests.

This can also be achieved with a pytest flag like -k "not dialog". But in this case the split makes sense anyway, because dialogs are a large and self-contained group of tests.