Voiceover hint for macOS DetailedList items isn't helpful

[freakboy3742]

Describe the bug

On macOS, the Voiceover assistant describes the content of DetailedList items as "<TogaData: 0xdeadbeef>", or similar. This isn't very helpful for navigation purposes.

Steps to reproduce

1. Run the DetailedList example
2. Turn on VoiceOver (Command-F5)
3. Select an item

The text that is read won't be the title and subtitle, but "<Togadata...>"

Expected behavior

The VoiceOver description should accurately describe the content of the list item

Screenshots

Environment

• Operating System: macOS
• Python version: All
• Software versions:
  • Toga: 0.4.2+

Logs

Additional context

The fix for this is to add an accessibilityValue() objc_method to the implementation of TogaDetailedCell. This is the macOS native implementation of an item in a detailedList; the default implementation is to read the description of the objectValue instance associated with the cell, which is a TogaData object.

We can reliably know that the objectValue will be a TogaData object with title and subtitle keys; these can be used to construct a description that can be read.

See #2233 for a related accessibility feature.