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/dom/scroll_indicator.cpp

@@ -0,0 +1,53 @@
+#include "ftxui/dom/elements.hpp"
+#include "ftxui/dom/node.hpp"
+#include "ftxui/dom/node_decorator.hpp"  // for NodeDecorator
+#include "ftxui/screen/box.hpp"
+#include "ftxui/screen/screen.hpp"
+
+namespace ftxui {
+
+/// @brief Add a filter that will invert the foreground and the background
+/// colors.
+/// @ingroup dom
+Element vscroll_indicator(Element child) {
+  class Impl : public NodeDecorator {
+    using NodeDecorator::NodeDecorator;
+
+    void Render(Screen& screen) final {
+      Node::Render(screen);
+
+      const Box& stencil = screen.stencil;
+
+      float size_inner = box_.y_max - box_.y_min;
+      float size_outter = stencil.y_max - stencil.y_min;
+      float start_y = stencil.y_min +
+                      (stencil.y_min - box_.y_min) * size_outter / size_inner;
+      float end_y = stencil.y_min +
+                    (stencil.y_max - box_.y_min) * size_outter / size_inner;
+
+      const int x = stencil.x_max;
+      for (int y = stencil.y_min; y <= stencil.y_max; ++y) {
+        bool up = (2 * y + -1 >= 2 * start_y) && (2 * y + -1 <= 2 * end_y);
+        bool down = (2 * y + 0 >= 2 * start_y) && (2 * y + 0 <= 2 * end_y);
+
+        if (up) {
+          if (down) {
+            screen.PixelAt(x, y).character = "┃";
+          } else {
+            screen.PixelAt(x, y).character = "╹";
+          }
+        } else {
+          if (down) {
+            screen.PixelAt(x, y).character = "╻";
+          } else {
+            screen.PixelAt(x, y).character = " ";
+          }
+        }
+        screen.PixelAt(x,y).inverted = true;
+      }
+    };
+  };
+  return std::make_shared<Impl>(std::move(child));
+}
+
+}  // namespace ftxui