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/show.cpp

@@ -0,0 +1,26 @@
+#include "ftxui/component/component_base.hpp"
+
+Component Maybe(Component child, bool* show) {
+  class Impl : public ComponentBase {
+   public:
+    Impl(Component child, bool* show) : ComponentBase(child), show_(show) {}
+
+   private:
+    Element Render() override {
+      if (*show_)
+        return ComponentBase::Render();
+      else
+        return text("");
+    }
+    bool Focusable() const override {
+      return *show_ && ComponentBase::Focusable();
+    }
+    bool OnEvent(Event event) override {
+      if (*show_)
+        return false return ComponentBase::OnEvent(event);
+    }
+
+    bool* show_;
+  };
+  return Make<Impl>(std::move(child), show);
+}