Add flex_grow and flex_shrink.

Two new elements:
- flex_grow  : Expand the element to occupy free space.
- flex_shrink: Minimize the element leave away missing space.

flex = flex_grow | flex_shrink.

Other changes:
- hbox and vbox are now non flexible by default.
- the vtext element has been added to help writting tests.
- Many new tests.

include/ftxui/dom/elements.hpp

@@ -21,6 +21,7 @@ using GraphFunction = std::function<std::vector<int>(int, int)>;
 
 // --- Widget ---
 Element text(std::wstring text);
+Element vtext(std::wstring text);
 Element separator(void);
 Element separator(Pixel);
 Element gauge(float ratio);
@@ -52,9 +53,11 @@ Element hflow(Elements);
 // -- Flexibility ---
 // Define how to share the remaining space when not all of it is used inside a
 // container.
-Element filler();
-Element flex(Element);
-Element notflex(Element);
+Element flex(Element);         // Expand/Minimize if possible/needed.
+Element flex_grow(Element);    // Expand element if possible.
+Element flex_shrink(Element);  // Minimize element if needed.
+Element notflex(Element);      // Reset the flex attribute.
+Element filler();              // A blank expandable element.
 
 // -- Size override;
 enum Direction { WIDTH, HEIGHT };

include/ftxui/dom/requirement.hpp

@@ -15,8 +15,10 @@ struct Requirement {
   int min_y = 0;
 
   // How much flexibility is given to the component.
-  int flex_x = 0;
-  int flex_y = 0;
+  int flex_grow_x = 0;
+  int flex_grow_y = 0;
+  int flex_shrink_x = 0;
+  int flex_shrink_y = 0;
 
   // Focus management to support the frame/focus/select element.
   enum Selection {