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/dropdown.cpp

@@ -0,0 +1,54 @@
+#include "ftxui/component/component.hpp"
+#include "ftxui/component/component_base.hpp"
+#include "ftxui/component/event.hpp"
+
+namespace ftxui {
+
+Component Dropdown(ConstStringListRef entries, int* selected) {
+  class Impl : public ComponentBase {
+   public:
+    Impl(ConstStringListRef entries, int* selected)
+        : entries_(std::move(entries)), selected_(selected) {
+      CheckboxOption option;
+      option.style_checked = "↓│";
+      option.style_unchecked = "→│";
+      checkbox_ = Checkbox(&title_, &show_, option),
+      radiobox_ = Radiobox(entries_, selected_);
+
+      Add(Container::Vertical({
+          checkbox_,
+          Maybe(radiobox_, &show_),
+      }));
+    }
+
+    Element Render() override {
+      title_ = entries_[*selected_];
+      if (show_) {
+        return vbox({
+                   checkbox_->Render(),
+                   separator(),
+                   radiobox_->Render() | vscroll_indicator | frame |
+                       size(HEIGHT, LESS_THAN, 12),
+               }) |
+               border;
+      }
+
+      return vbox({
+          checkbox_->Render() | border,
+          filler(),
+      });
+    }
+
+   private:
+    ConstStringListRef entries_;
+    bool show_ = false;
+    int* selected_;
+    std::string title_;
+    Component checkbox_;
+    Component radiobox_;
+  };
+
+  return Make<Impl>(std::move(entries), selected);
+}
+
+}  // namespace ftxui