Implemented bobura::main_window.
Implemented bobura::settings.
Implemented tetengo2::gui::widget::abstract_window.
Implemented bobura::message::main_window.
Modified stdlib_headers.txt.
Removed and added TODO items.
Corrected compilation errors.
Implemented tetengo2::gui::message::mouse_observer_set.
Implemented tetengo2::detail::windows::message_handler_detail::widget.
Removed unnecessary namespace decorations.