utility: imported duct::bit(); updated bit enums; doc tidy.
ui::Context: moved focus handling to ui::Root, removed ^C handler.

ui::Context:
	added get_last_event();
	update(): return whether event was handled;
	update(): push events to root if no widget is focused.
ui::Widget::Base: added signal_event_filter.¹

1: To enable changing widget behavior without extension/replacement.
ui::Widget::Base: exit render if widget is not visible.