Add support for nxxm.

[nxxm](https://nxxm.github.io)

include/ftxui/component/checkbox.hpp

@@ -0,0 +1,39 @@
+#ifndef FTXUI_COMPONENT_CHECKBOX_HPP
+#define FTXUI_COMPONENT_CHECKBOX_HPP
+
+#include "ftxui/component/component.hpp"
+#include <functional>
+
+namespace ftxui {
+
+class CheckBox : public Component {
+ public:
+  // Constructor.
+  CheckBox() = default;
+  ~CheckBox() override = default;
+
+  bool state = false;
+  std::wstring label = L"label";
+
+  //std::wstring checked = L"[X] ";
+  //std::wstring unchecked = L"[ ] ";
+  std::wstring checked = L"▣ ";
+  std::wstring unchecked = L"☐ ";
+
+  Decorator focused_style = inverted;
+  Decorator unfocused_style = nothing;
+
+  // State update callback.
+  std::function<void()> on_change = [](){};
+
+  // Component implementation.
+  Element Render() override;
+  bool OnEvent(Event) override;
+
+ private:
+  int cursor_position = 0;
+};
+
+}  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_COMPONENT_CHECKBOX_HPP */