Factorize box layout functions. (#185)

|hbox| and |vbox| are similar. They are just the same component, but
drawing in a different direction.

This patchs factorize the layout logic. Goal is to reduce code size and
reuse it for the |gridbox| dom element.

Bug: https://github.com/ArthurSonzogni/FTXUI/issues/114

src/ftxui/dom/box_helper.hpp

@@ -0,0 +1,24 @@
+#ifndef FTXUI_DOM_BOX_HELPER_HPP
+#define FTXUI_DOM_BOX_HELPER_HPP
+
+#include <vector>
+
+namespace ftxui {
+namespace box_helper {
+
+struct Element {
+  // Input:
+  int min_size = 0;
+  int flex_grow = 0;
+  int flex_shrink = 0;
+
+  // Output;
+  int size = 0;
+};
+
+void Compute(std::vector<Element>* elements, int target_size);
+
+}  // namespace box_helper
+}  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_DOM_BOX_HELPER_HPP */