A map from each supported CustomSemanticsAction to a provided handler.
The handler associated with each custom action is called whenever a
semantics event of type
SemanticsEvent.customEvent is received. The
provided argument will be an identifier used to retrieve an instance of
a custom action which can then retrieve the correct handler from this map.
- CustomSemanticsAction, for an explanation of custom actions.
final Map<CustomSemanticsAction, VoidCallback> customSemanticsActions