Feature: Slider in any directions. (#468)

Add the `SliderOption` option supporting:
```cpp
{
  Ref<T> value;
  ConstRef<T> min = T(0);
  ConstRef<T> max = T(100);
  ConstRef<T> increment = (max() - min()) / 20;
  GaugeDirection direction = GaugeDirection::Right;
  Color color_active = Color::White;
  Color color_inactive = Color::GrayDark;
};
```

In particular, this supports multiple direction. This resolves:
https://github.com/ArthurSonzogni/FTXUI/issues/467

This one do not support adding a label. The old constructors can still
be used to have a label.

examples/CMakeLists.txt

@@ -4,6 +4,9 @@ function(example name)
   target_link_libraries(ftxui_example_${name} PUBLIC ${DIRECTORY_LIB})
   file(RELATIVE_PATH dir ${EXAMPLES_DIR} ${CMAKE_CURRENT_SOURCE_DIR})
   set_property(GLOBAL APPEND PROPERTY FTXUI::EXAMPLES ${dir}/${name})
+  set_target_properties(ftxui_example_${name} PROPERTIES
+    CXX_STANDARD 20
+  )
 endfunction(example)
 
 add_subdirectory(component)

examples/component/CMakeLists.txt

@@ -33,6 +33,7 @@ example(radiobox_in_frame)
 example(renderer)
 example(resizable_split)
 example(slider)
+example(slider_direction)
 example(slider_rgb)
 example(tab_horizontal)
 example(tab_vertical)

examples/component/slider_direction.cpp

@@ -0,0 +1,37 @@
+#include <array>                               // for array
+#include <cmath>                               // for sin
+#include <ftxui/component/component_base.hpp>  // for ComponentBase
+#include <ftxui/dom/elements.hpp>  // for size, GaugeDirection, GaugeDirection::Up, GREATER_THAN, HEIGHT
+#include <memory>  // for shared_ptr, __shared_ptr_access
+
+#include "ftxui/component/captured_mouse.hpp"  // for ftxui
+#include "ftxui/component/component.hpp"  // for Horizontal, Slider, operator|=
+#include "ftxui/component/screen_interactive.hpp"  // for ScreenInteractive
+
+using namespace ftxui;
+
+int main(int argc, const char* argv[]) {
+  auto screen = ScreenInteractive::TerminalOutput();
+  std::array<int, 30> values;
+  for (int i = 0; i < values.size(); ++i) {
+    values[i] = 50 + 20 * std::sin(i * 0.3);
+  }
+
+  auto layout_horizontal = Container::Horizontal({});
+  for (int i = 0; i < values.size(); ++i) {
+    layout_horizontal->Add(Slider<int>({
+        .value = &values[i],
+        .max = 100,
+        .increment = 5,
+        .direction = GaugeDirection::Up,
+    }));
+  }
+
+  layout_horizontal |= size(HEIGHT, GREATER_THAN, 20);
+
+  screen.Loop(layout_horizontal);
+}
+
+// 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.