Implement a lot of new features.

This commit deserve to be cut into at least 8 sub commit. Sorry, I
acknowledge this is bad... Here are the new features:

 * dom decorator: bold, dim, underlined, inverted.
 * component mechanism
 * components
   * menu
   * toogle

examples/print_key_press/CMakeLists.txt

@@ -0,0 +1,4 @@
+add_executable(print_key_press
+  main.cpp
+)
+target_link_libraries(print_key_press PRIVATE ftxui)

examples/print_key_press/main.cpp

@@ -0,0 +1,44 @@
+#include <chrono>
+#include <iostream>
+#include <thread>
+
+#include "ftxui/screen_interactive.hpp"
+#include "ftxui/component/component.hpp"
+#include "ftxui/util/string.hpp"
+
+class DrawKey : public ftxui::component::Component {
+ public:
+  DrawKey(ftxui::component::Delegate* delegate)
+      : ftxui::component::Component(delegate) {}
+
+  ftxui::dom::Element Render() override {
+    using namespace ftxui::dom;
+    Children children;
+    for (size_t i = std::max(0, (int)keys.size() - 10); i < keys.size(); ++i) {
+      try {
+        std::string line = std::to_string(i) + " -> " + std::to_string(keys[i]) +
+                           " (" + char(keys[i]) + ")";
+        children.push_back(text(to_wstring(line)));
+      } catch (...) {
+        std::string line = std::to_string(i) + " -> " + std::to_string(keys[i]) +
+                           " (undefined)";
+        children.push_back(text(to_wstring(line)));
+      }
+    }
+    return vbox(std::move(children));
+  }
+
+  bool Event(int key) override {
+    keys.push_back(key);
+    return true;
+  }
+
+ private:
+  std::vector<int> keys;
+};
+
+int main(int argc, const char* argv[]) {
+  ftxui::ScreenInteractive screen(80,10);
+  DrawKey draw_key(screen.delegate());
+  screen.Loop();
+}