Ensure that the top bar size is included in iOS layout calculations.

[freakboy3742]

As part of the introduction of simple apps in #2649, the handling of containers on iOS was refactored to introduce NavigationContainer (which has a navigation bar) as distinct from RootContainer (which doesn't). However, this refactor inadvertently altered the calculation of the available height of the container in the NavigationContainer case to not include the height of the navigation bar itself. The mechansim to compute the top bar height was present - but due to inheritance, the top bar height wasn't subtracted from the container height.

This restores the use of top bar height in container calculations, and adds a testbed test to ensure that content fits in the available area in mobile layouts.

Fixes #2836.

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

[jlij]

Hello, has this issue been fixed?
If resolved, how should I use it.

[mhsmith]

Yes, it was released in Toga 0.4.7. Make sure the Toga requirements in your pyproject.toml file either use this version, or a version range that includes it, and then run briefcase build ios -ur.