Implement flexbox (#277)

This implement the flexbox elements, following the HTML one.

Built from them, there is also the following elements:
- `paragraph`
- `paragraphAlignLeft`
- `paragraphAlignRight`
- `paragraphAlignCenter`
- `paragraphAlignJustify`

This is a breaking change.

examples/component/checkbox.cpp

@@ -1,6 +1,12 @@
+#include <memory>  // for allocator, __shared_ptr_access
+#include <string>  // for string, basic_string, operator+, to_string
+#include <vector>  // for vector
+
+#include "ftxui/component/captured_mouse.hpp"  // for ftxui
+#include "ftxui/component/component.hpp"       // for Input, Renderer, Vertical
+#include "ftxui/component/component_base.hpp"  // for ComponentBase
 #include "ftxui/component/screen_interactive.hpp"  // for Component, ScreenInteractive
-#include "ftxui/dom/elements.hpp"
-#include "ftxui/component/component.hpp"
+#include "ftxui/dom/elements.hpp"  // for operator|, Element, size, border, frame, vscroll_indicator, HEIGHT, LESS_THAN
 
 int main(int argc, const char* argv[]) {
   using namespace ftxui;
@@ -19,3 +25,7 @@ int main(int argc, const char* argv[]) {
   auto screen = ScreenInteractive::TerminalOutput();
   screen.Loop(renderer);
 }
+
+// Copyright 2021 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.