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

src/ftxui/component/maybe.cpp

@@ -0,0 +1,31 @@
+#include "ftxui/component/component.hpp"
+#include "ftxui/component/component_base.hpp"
+#include "ftxui/component/event.hpp"
+
+namespace ftxui {
+
+Component Maybe(Component child, bool* show) {
+  class Impl : public ComponentBase {
+   public:
+    Impl(bool* show): show_(show) {}
+
+   private:
+    Element Render() override {
+      return *show_ ? ComponentBase::Render() : std::make_unique<Node>();
+    }
+    bool Focusable() const override {
+      return *show_ && ComponentBase::Focusable();
+    }
+    bool OnEvent(Event event) override {
+      return *show_ && ComponentBase::OnEvent(event);
+    }
+
+    bool* show_;
+  };
+  
+  auto maybe = Make<Impl>(show);
+  maybe->Add(std::move(child));
+  return maybe;
+}
+
+}  // namespace ftxui