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

examples/component/CMakeLists.txt

@@ -10,6 +10,7 @@ example(gallery)
 example(homescreen)
 example(input)
 example(maybe)
+example(focus)
 example(menu)
 example(menu2)
 example(menu_entries)

examples/component/focus.cpp

@@ -0,0 +1,72 @@
+#include <stddef.h>  // for size_t
+#include <memory>    // for shared_ptr, __shared_ptr_access, allocator
+#include <string>  // for string, basic_string, to_string, operator+, char_traits
+#include <vector>  // for vector
+
+#include "ftxui/component/captured_mouse.hpp"  // for ftxui
+#include "ftxui/component/component.hpp"  // for Radiobox, Vertical, Checkbox, Horizontal, Renderer, ResizableSplitBottom, ResizableSplitRight
+#include "ftxui/component/component_base.hpp"      // for ComponentBase
+#include "ftxui/component/screen_interactive.hpp"  // for ScreenInteractive
+#include "ftxui/dom/elements.hpp"  // for text, window, operator|, vbox, hbox, Element, flexbox, bgcolor, filler, flex, size, border, hcenter, color, EQUAL, bold, dim, notflex, xflex_grow, yflex_grow, HEIGHT, WIDTH
+#include "ftxui/dom/flexbox_config.hpp"  // for FlexboxConfig, FlexboxConfig::AlignContent, FlexboxConfig::JustifyContent, FlexboxConfig::AlignContent::Center, FlexboxConfig::AlignItems, FlexboxConfig::Direction, FlexboxConfig::JustifyContent::Center, FlexboxConfig::Wrap
+#include "ftxui/screen/color.hpp"        // for Color, Color::Black
+
+using namespace ftxui;
+
+Element make_box(int x, int y) {
+  std::string title = "(" + std::to_string(x) + ", " + std::to_string(y) + ")";
+  return text(title) | center | size(WIDTH, EQUAL, 18) |
+         size(HEIGHT, EQUAL, 9) | border |
+         bgcolor(Color::HSV(x * 255 / 15, 255, y * 255 / 15));
+};
+
+Element make_grid() {
+  std::vector<Elements> rows;
+  for (int i = 0; i < 15; i++) {
+    std::vector<Element> cols;
+    for (int j = 0; j < 15; j++) {
+      cols.push_back(make_box(i, j));
+    }
+    rows.push_back(cols);
+  }
+
+  return gridbox(rows);
+};
+
+int main(int argc, const char* argv[]) {
+  float focus_x = 0.0f;
+  float focus_y = 0.0f;
+
+  auto slider_x = Slider("x", &focus_x, 0.f, 1.f, 0.05f);
+  auto slider_y = Slider("y", &focus_y, 0.f, 1.f, 0.05f);
+
+  auto renderer = Renderer(
+      Container::Vertical({
+          slider_x,
+          slider_y,
+      }),
+      [&] {
+        auto title = "focusPositionRelative(" +        //
+                     std::to_string(focus_x) + ", " +  //
+                     std::to_string(focus_y) + ")";    //
+        return vbox({
+                   text(title),
+                   separator(),
+                   slider_x->Render(),
+                   slider_y->Render(),
+                   separator(),
+                   make_grid() | focusPositionRelative(focus_x, focus_y) |
+                       frame | flex,
+               }) |
+               border;
+      });
+
+  auto screen = ScreenInteractive::Fullscreen();
+  screen.Loop(renderer);
+
+  return 0;
+}
+
+// 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.