feat: Dropdown select menu. (#214)

Dom
 - `vscroll_indicator`. Show a scrollback indicator on the right.

Component
 - `Maybe`: Display an component conditionnally based on a boolean.
 - `Dropdown`: A dropdown select list.

This address:
https://github.com/ArthurSonzogni/FTXUI/issues/204

examples/component/maybe.cpp

@@ -0,0 +1,47 @@
+#include <functional>  // for function
+#include <iostream>  // for basic_ostream::operator<<, operator<<, endl, basic_ostream, basic_ostream<>::__ostream_type, cout, ostream
+#include <string>    // for string, basic_string, allocator
+#include <vector>    // for vector
+
+#include "ftxui/component/captured_mouse.hpp"      // for ftxui
+#include "ftxui/component/component.hpp"           // for Menu
+#include "ftxui/component/component_options.hpp"   // for MenuOption
+#include "ftxui/component/screen_interactive.hpp"  // for ScreenInteractive
+
+using namespace ftxui;
+Component Border(Component child) {
+  return Renderer(child, [child] { return child->Render() | border; });
+}
+
+int main(int argc, const char* argv[]) {
+
+  std::vector<std::string> entries = {
+      "entry 1",
+      "entry 2",
+      "entry 3",
+  };
+  int menu_1_selected = 0;
+  int menu_2_selected = 0;
+  auto menu_1 = Radiobox(&entries, &menu_1_selected);
+  auto menu_2 = Radiobox(&entries, &menu_2_selected);
+
+  menu_1 = Border(menu_1);
+  menu_2 = Border(menu_2);
+
+  bool menu_1_show = false;
+  bool menu_2_show = false;
+
+  auto layout = Container::Vertical({
+      Checkbox("Show menu_1", &menu_1_show),
+      Maybe(menu_1, &menu_1_show),
+      Checkbox("Show menu_2", &menu_2_show),
+      Maybe(menu_2, &menu_2_show),
+  });
+
+  auto screen = ScreenInteractive::TerminalOutput();
+  screen.Loop(layout);
+}
+
+// Copyright 2020 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.