Add "frame" : scrollable area.

README.md

examples/component/CMakeLists.txt

examples/component/checkbox.cpp

examples/component/checkbox_in_frame.cpp

examples/component/gallery.cpp

examples/component/input.cpp

examples/component/menu2.cpp

examples/component/menu_style.cpp

examples/component/radiobox_in_frame.cpp

examples/component/tab_horizontal.cpp

examples/dom/CMakeLists.txt

examples/dom/border.cpp

examples/dom/dbox.cpp

examples/dom/spinner.cpp

ftxui/CMakeLists.txt

ftxui/include/ftxui/component/container.hpp

ftxui/include/ftxui/component/menu.hpp

ftxui/include/ftxui/component/toggle.hpp

ftxui/include/ftxui/dom/box.hpp

ftxui/include/ftxui/dom/elements.hpp