Add gauge and frame.

examples/CMakeLists.txt

examples/frame/CMakeLists.txt

examples/frame/main.cpp

examples/gauge/main.cpp

ftxui/CMakeLists.txt

ftxui/include/ftxui/core/dom/elements.hpp

ftxui/src/ftxui/core/dom/frame.cpp

ftxui/src/ftxui/core/dom/gauge.cpp

ftxui/src/ftxui/core/dom/vbox.cpp