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/menu2/CMakeLists.txt

@@ -0,0 +1,4 @@
+add_executable(menu2_main
+  main.cpp
+)
+target_link_libraries(menu2_main PRIVATE ftxui)

examples/menu2/main.cpp

@@ -0,0 +1,75 @@
+#include <chrono>
+#include <iostream>
+#include <thread>
+
+#include "ftxui/screen_interactive.hpp"
+#include "ftxui/component/menu.hpp"
+#include "ftxui/component/component_horizontal.hpp"
+#include "ftxui/component/component_vertical.hpp"
+#include "ftxui/util/string.hpp"
+
+using namespace ftxui::component;
+using namespace ftxui::dom;
+
+class MyComponent : ComponentHorizontal {
+  public:
+   MyComponent(ftxui::component::Delegate* delegate)
+       : ComponentHorizontal(delegate),
+         left_menu(delegate->NewChild()),
+         right_menu(delegate->NewChild()) {
+     left_menu.entries = {L"0%",  L"10%", L"20%", L"30%", L"40%", L"50%",
+                          L"60%", L"70%", L"80%", L"90%"};
+     right_menu.entries = {L"0%",  L"1%", L"2%", L"3%", L"4%", L"5%",
+                           L"6%", L"7%", L"8%", L"9%", L"10%"};
+
+     left_menu.on_enter = [this]() { on_enter(); };
+     right_menu.on_enter = [this]() { on_enter(); };
+     Focus(&left_menu);
+   }
+
+   std::function<void()> on_enter = [](){};
+  private:
+   Menu left_menu;
+   Menu right_menu;
+
+   Element Render() override {
+     int sum = left_menu.selected * 10 + right_menu.selected;
+     return
+       frame(
+         vbox(
+           // -------- Top panel --------------
+           hbox(
+             // -------- Left Menu --------------
+             flex(
+               vbox(
+                 hcenter(bold(text(L"Percentage by 10%"))),
+                 left_menu.Render()
+               )
+             ),
+             // -------- Right Menu --------------
+             flex(
+               vbox(
+                 hcenter(bold(text(L"Percentage by 1%"))),
+                 right_menu.Render()
+               )
+             ),
+             flex()
+           ),
+           separator(),
+           // -------- Bottom panel --------------
+           flex(vbox(
+             hbox(text(L" gauge : "), gauge(sum/100.0)),
+             hbox(text(L"  text : "), text(to_wstring(std::to_string(sum) + " %")))
+           ))
+         )
+       );
+   }
+};
+
+int main(int argc, const char *argv[])
+{
+  ftxui::ScreenInteractive screen(60,17);
+  MyComponent component(screen.delegate());
+  component.on_enter = screen.ExitLoopClosure();
+  screen.Loop();
+}