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/separator/main.cpp

@@ -1,5 +1,5 @@
-#include "ftxui/core/screen.hpp"
-#include "ftxui/core/dom/elements.hpp"
+#include "ftxui/screen.hpp"
+#include "ftxui/dom/elements.hpp"
 #include <iostream>
 
 int main(int argc, const char *argv[])
@@ -15,7 +15,7 @@ int main(int argc, const char *argv[])
         center(text(L"bottom-column"))
       ))
     );
-  auto screen = ftxui::Screen::WholeTerminal();
+  auto screen = ftxui::Screen::TerminalFullscreen();
   Render(screen, document.get());
 
   std::cout << screen.ToString();