Adds an ActivateAction to controls that use InkWell. Make InkWell host the Focus for those controls, and add the top level binding to the ENTER key. This will make it possible to trigger a button using the enter key, and to get an ink response when the button is triggered.

This is a breaking change because it moves the Focus widget into the InkWell. If you have a component that uses the InkWell directly and you used to wrap that InkWell in a Focus widget to give it its notion of focus, it will now not look for that Focus ancestor for its focus state anymore. In order to control focus on the InkWell, you need to give it a FocusNode directly, via the new focusNode parameter. This should not affect users of widgets like OutlineButton or FloatingActionButton and the like, since those have been modified in this PR.

This re-lands #41417 with a slight change that will hopefully not tickle the analyzer as it did before. The last time I tried to land this, the analyzer succeeded for the analyze step in Cirrus, and locally, but failed in an integration test.

This reverts commit eb3e2f55 because of a suspected analyzer bug.  Reverting to investigate.

This addresses comments in the original PR (#41245) that introduced Intent.doNothing, adds tests, and fixes an issue with it.

This changes the way ActionDispatchers are found by the Actions widget, so that by default it will look for dispatchers of the parent Actions widgets instead of just creating a default ActionDispatcher. This allows overriding of the ActionDispatcher at the top level: before, the custom action dispatcher would only be invoked if explicitly set on all the Actions widgets.

This is not a breaking change because there was a default value to the dispatcher parameter before that performed this function, and not specifying the dispatcher anywhere will still result in a default dispatcher being created.

This implements the keyboard shortcut handling and action invocation in order to provide a place in the infrastructure for keyboard events to trigger actions. This will allow binding of key events to actions like "move the focus to the next widget" and "activate button".