Similar to the mouse_grabber, we use a VisitChildrenResult field to
track the focus item within a component. Unlike the mouse grabber
however, it is set/cleared using dedicated focus events.
The key event now routes the key event directly to the focus item.
The focus can be requested via set_focus_item on a window, which the
TextItem does.
In the future the TextInput will request focus on mouse click,for
example.
Pass the outer-most component through to ItemVTable's input_event.
For the purpose of disambiguating this component from any nested
component instantiated by a repeater or so, it's called the
app_component.
The ComponentVTable takes a reference to a ComponentRefPin instead of a
ComponentRefPin by value, as the vtable macro gets confused otherwise
and thinks it's a self argument.
