ui: corrected focus debug rendering.
Revised clear_actions() in ui::Context and ui::Widget::Base; doc tidy.

ui::Widget::Base: added option to dequeue in clear_actions().

ui::Context: always enqueue; revised clear_actions().

ui::Button: doc tidy.
Moved debug cell frames to tty/Defs; framed ui::geom_debug_render() by focus.
ui: added Label widget; added test in test/ui/packing.
Aliased std::function (aux) and duct::char32 (String); doc tidy.
tty::Sequence: added position for String ctor; doc tidy & tidy.