This is impossible with the new message model. No big loss.

This is not emitted by their children

This breaks ComboBox and automatic updating of data
through view widgets (to be fixed later).

This is a big revision to event handling, allowing multiple
simultaneous messages from children and removing the need to
parametrize many widgets over the message type.

Incomplete: new Handler::on_message method not called yet.

Remove Response::Msg, VoidMsg, #[derive(VoidMsg)]
Replace event::ChildMsg with IndexMsg, SelectMsg
Remove Updatable trait; add SingleData::on_message, etc.
Remove #[widget(flatmap_msg=.., map_msg=.., use_msg=.., discard_msg)]
make_widget!: remove -> message-type bound
AdaptWidget: remove map_void_msg, map_msg_discard; adjust map_msg
Button, etc...: event closure no longer returns Option<M>
Grid, List: now push IndexMsg given a child message

Rename widgets::view::SelectMsg → SelectionMsg

New Scroll enum, EventMgr::set_scroll and Handler::scroll methods

This gets lots of things working again, but many bugs remain.

This prevents keys activating by combobox menu items from
doing anything unless the menu is already open, which is
the same behaviour as used by menus.

This enables useful output in the case of unhandled messages.

Slider now holds handle to its detents.

Slider now takes nav-focus on click.

Same for SingleData, ListData, MatrixData

This avoids unwanted sending of IndexMsg and is more flexible.
Fixes light/dark theme buttons in the gallery (embedded in a Row).

…urning UpdateHandle