Add support for nxxm.

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

src/ftxui/component/container.cpp

@@ -0,0 +1,108 @@
+#include "ftxui/component/container.hpp"
+
+namespace ftxui {
+
+// static
+Container Container::Horizontal() {
+  Container container;
+  container.event_handler_ = &Container::HorizontalEvent;
+  container.render_handler_ = &Container::HorizontalRender;
+  return container;
+}
+
+// static
+Container Container::Vertical() {
+  Container container;
+  container.event_handler_ = &Container::VerticalEvent;
+  container.render_handler_ = &Container::VerticalRender;
+  return container;
+}
+
+// static
+Container Container::Tab(int* selector) {
+  Container container;
+  container.event_handler_ = &Container::TabEvent;
+  container.render_handler_ = &Container::TabRender;
+  container.selector_ = selector;
+  return container;
+}
+
+bool Container::OnEvent(Event event) {
+  if (!Focused())
+    return false;
+
+  if (ActiveChild()->OnEvent(event))
+    return true;
+
+  return (this->*event_handler_)(event);
+}
+
+Component* Container::ActiveChild() {
+  return children_[*selector_ % children_.size()];
+}
+
+bool Container::VerticalEvent(Event event) {
+  selected_ %= children_.size();
+  // Left pressed ?
+  if (event == Event::ArrowUp || event == Event::Character('k')) {
+    if (selected_ != 0) {
+      selected_--;
+      return true;
+    }
+  }
+
+  // Left pressed ?
+  if (event == Event::ArrowDown || event == Event::Character('j')) {
+    if (selected_ != int(children_.size()) - 1) {
+      selected_++;
+      return true;
+    }
+  }
+
+  return false;
+}
+
+bool Container::HorizontalEvent(Event event) {
+  selected_ %= children_.size();
+  // Left pressed ?
+  if (event == Event::ArrowLeft || event == Event::Character('h')) {
+    if (selected_ != 0) {
+      selected_--;
+      return true;
+    }
+  }
+
+  // Left pressed ?
+  if (event == Event::ArrowRight || event == Event::Character('l')) {
+    if (selected_ != int(children_.size()) - 1) {
+      selected_++;
+      return true;
+    }
+  }
+
+  return false;
+}
+
+Element Container::Render() {
+  return (this->*render_handler_)();
+}
+
+Element Container::VerticalRender() {
+  Elements elements;
+  for(auto& it : children_)
+    elements.push_back(it->Render());
+  return vbox(std::move(elements));
+}
+
+Element Container::HorizontalRender() {
+  Elements elements;
+  for(auto& it : children_)
+    elements.push_back(it->Render());
+  return hbox(std::move(elements));
+}
+
+Element Container::TabRender() {
+  return ActiveChild()->Render();
+}
+
+}  // namespace ftxui