Check for Semantics to verify and improve screen reader support

[MrCsabaToth]

Is your feature request related to a problem? Please describe.
An application rating mentioned difficulties from a user using screen reader. We should verify that the widget's Semantics are proper.

Describe the solution you'd like
Key concepts for semantic testing

• Semantics tree: Flutter maintains a separate tree of semantic information alongside the normal widget tree. This tree is what assistive technologies, such as screen readers, use to describe the UI to users.
• Semantics widget: You use the Semantics widget to explicitly add semantic information to your custom widgets, describing their purpose for accessibility tools.
• WidgetTester.getSemantics(): This method is the core of automated semantic testing. It retrieves the semantic information for a specific widget in the tree, allowing you to run checks on it.
• matchesSemantics(): A helper matcher from the flutter_test library that simplifies comparing the semantics of a widget against a predefined description.
  Describe alternatives you've considered

[MrCsabaToth]

See also CsabaConsulting/listview_utils_plus#3