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

include/ftxui/component/component.hpp

@@ -39,6 +39,7 @@ Component Menu(ConstStringListRef entries,
                int* selected_,
                Ref<MenuOption> = {});
 Component MenuEntry(ConstStringRef label, Ref<MenuEntryOption> = {});
+Component Dropdown(ConstStringListRef entries, int* selected);
 Component Radiobox(ConstStringListRef entries,
                    int* selected_,
                    Ref<RadioboxOption> option = {});
@@ -55,6 +56,7 @@ Component Renderer(Component child, std::function<Element()>);
 Component Renderer(std::function<Element()>);
 Component Renderer(std::function<Element(bool /* focused */)>);
 Component CatchEvent(Component child, std::function<bool(Event)>);
+Component Maybe(Component, bool* show);
 
 namespace Container {
 Component Vertical(Components children);

include/ftxui/component/event.hpp

@@ -74,6 +74,7 @@ struct Event {
   bool operator!=(const Event& other) const { return !operator==(other); }
 
   //--- State section ----------------------------------------------------------
+  ScreenInteractive* screen_ = nullptr;
  private:
   friend ComponentBase;
   friend ScreenInteractive;
@@ -95,8 +96,6 @@ struct Event {
     struct Cursor cursor_;
   };
   std::string input_;
-
-  ScreenInteractive* screen_ = nullptr;
 };
 
 }  // namespace ftxui

include/ftxui/dom/elements.hpp

@@ -104,6 +104,8 @@ Element yframe(Element);
 Element focus(Element);
 Element select(Element);
 
+Element vscroll_indicator(Element);
+
 // --- Util --------------------------------------------------------------------
 Element hcenter(Element);
 Element vcenter(Element);