Add focusPosition[relative](x,y) (#280)

It allows when using inside a frame, to scroll the view toward a
particular position.

This resolves:
https://github.com/ArthurSonzogni/FTXUI/issues/125

src/ftxui/dom/focus.cpp

@@ -0,0 +1,103 @@
+#include <memory>   // for make_shared
+#include <utility>  // for move
+
+#include "ftxui/dom/elements.hpp"        // for Element, bold
+#include "ftxui/dom/node.hpp"            // for Node
+#include "ftxui/dom/node_decorator.hpp"  // for NodeDecorator
+#include "ftxui/screen/box.hpp"          // for Box
+#include "ftxui/screen/screen.hpp"       // for Pixel, Screen
+
+namespace ftxui {
+
+namespace {
+
+}  // namespace
+
+/// @brief Used inside a `frame`, this force the view to be scrolled toward a
+/// a given position. The position is expressed in proportion of the requested
+/// size.
+/// 
+/// For instance:
+/// - (0, 0) means that the view is scrolled toward the upper left.
+/// - (1, 0) means that the view is scrolled toward the upper right.
+/// - (0, 1) means that the view is scrolled toward the bottom left.
+/// @ingroup dom
+///
+/// ### Example
+///
+/// ```cpp
+/// Element document = huge_document()
+///   | focusPositionRelative(0.f, 1.f)
+///   | frame;
+/// ```
+Decorator focusPositionRelative(float x, float y) {
+  class Impl : public NodeDecorator {
+   public:
+    Impl(Element child, float x, float y)
+        : NodeDecorator(child), x_(x), y_(y) {}
+
+    void ComputeRequirement() override {
+      NodeDecorator::ComputeRequirement();
+      requirement_.selection = Requirement::Selection::NORMAL;
+
+      Box& box = requirement_.selected_box;
+      box.x_min = requirement_.min_x * x_;
+      box.y_min = requirement_.min_y * y_;
+      box.x_max = requirement_.min_x * x_;
+      box.y_max = requirement_.min_y * y_;
+    }
+
+   private:
+    const float x_;
+    const float y_;
+  };
+
+  return [x, y](Element child) {
+    return std::make_shared<Impl>(std::move(child), x, y);
+  };
+}
+
+/// @brief Used inside a `frame`, this force the view to be scrolled toward a
+/// a given position. The position is expressed in the numbers of cells.
+/// 
+/// @ingroup dom
+///
+/// ### Example
+///
+/// ```cpp
+/// Element document = huge_document()
+///   | focusPosition(10, 10)
+///   | frame;
+/// ```
+Decorator focusPosition(int x, int y) {
+  class Impl : public NodeDecorator {
+   public:
+    Impl(Element child, float x, float y)
+        : NodeDecorator(child), x_(x), y_(y) {}
+
+    void ComputeRequirement() override {
+      NodeDecorator::ComputeRequirement();
+      requirement_.selection = Requirement::Selection::NORMAL;
+
+      Box& box = requirement_.selected_box;
+      box.x_min = x_;
+      box.y_min = y_;
+      box.x_max = x_;
+      box.y_max = y_;
+    }
+
+   private:
+    const int x_;
+    const int y_;
+  };
+
+  return [x, y](Element child) {
+    return std::make_shared<Impl>(std::move(child), x, y);
+  };
+}
+
+}  // namespace ftxui
+
+// Copyright 2020 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.