Add Widget::update; use input data

[dhardy]

This builds on #395, making use of input data.

Significant changes:

• Add Action::EVENT_CONFIG, Action::UPDATE; remove Action::EMPTY
• Add Events::update, fn EventState::request_update; call from configure and ConfigMgr::update; remove data from Events::configure; remove Event::Update
• Basic support for limiting recursion of updates with fn ConfigMgr::restrict_recursion_to
• Rename Events::handle_message → Events::handle_messages
• Alternative (now default) implementation of Node/NodeMut using dyn traits instead of unsafe
• Add Node::for_child, for_children, for_children_try, for_path
• Add ErasedStack and ability to send messages via the proxy
• Alternative configuration tweaking mechanism: fn EventState::change_config and configuration widget
• Move all of kas::model to kas-view; revise data traits, Driver trait, list filter mechanism; remove SingleView and revise ListData and MatrixData to use input data
• New widgets: Adapt, Map, OnUpdate
• Revise widget library to input data; revise EditGuard and impls
• Update examples to use input data; async-event is re-written, sync-counter more complex (global + per-window state), several are significantly shorter (especially counter, calculator, times-tables); others like clock and cursors and not affected much.

Overall the "input data" approach remains very performant, provides a more complete and integrated experience to ListView / MatrixView, and allows significantly less boilerplate in many example UIs.

There remain a few less-than-ideal items: inability to pass messages to children (e.g. the Gallery's filter list copies both selection mode and filter string to data since this is currently the only provided method of passing data to children); menus possibly need a redesign.

[dhardy]

Design document (belatedly) posted: kas-gui/design#6