Feature: Slider in any directions. (#468)

Add the `SliderOption` option supporting:
```cpp
{
  Ref<T> value;
  ConstRef<T> min = T(0);
  ConstRef<T> max = T(100);
  ConstRef<T> increment = (max() - min()) / 20;
  GaugeDirection direction = GaugeDirection::Right;
  Color color_active = Color::White;
  Color color_inactive = Color::GrayDark;
};
```

In particular, this supports multiple direction. This resolves:
https://github.com/ArthurSonzogni/FTXUI/issues/467

This one do not support adding a label. The old constructors can still
be used to have a label.

cmake/ftxui_test.cmake

@@ -34,6 +34,7 @@ add_executable(tests
   src/ftxui/component/modal_test.cpp
   src/ftxui/component/radiobox_test.cpp
   src/ftxui/component/receiver_test.cpp
+  src/ftxui/component/slider_test.cpp
   src/ftxui/component/resizable_split_test.cpp
   src/ftxui/component/screen_interactive_test.cpp
   src/ftxui/component/terminal_input_parser_test.cpp