Trait revision part 1:

[dhardy]

• Move kas::event::Handler methods into Widget; remove Handler trait
• Move kas::WidgetConfig methods into Widget; remove WidgetConfig trait
• Support merging properties defined via #[widget] attribute into Widget implementation and to override methods generated by derive
• Move some Layout methods to Widget trait: translation, spatial_nav, find_id
• Update kas-text: Breaking: return Result<T, NotReady> instead of panic kas-text#65
• Remove List::size_solved flag: do not inhibit drawing between adding new widgets and calling set_rect
• DrawMgr::with_clip_region and with_overlay: change closure type to FnOnce
• Add WidgetId field to DrawMgr; remove parameter from Component::draw
• Replace some explicit impls of Layout::layout (method is to be removed in the future)

Revision of the Layout trait is cut short here, but this PR is already large enough.