Add size(direction, constraint, value).

For example:
============

element
  | size(WIDTH, EQUAL 10);

element
  | size(HEIGHT, GREATER_THAN, 10);

element
  | size(WIDTH, EQUAL, 10)
  | size(HEIGHT, EQUAL, 10)

examples/component/checkbox_in_frame.cpp

@@ -25,7 +25,10 @@ class MyComponent : public Component {
     for(auto& it : checkbox) {
       content.push_back(it.Render());
     }
-    return vbox(std::move(content)) | frame | size(20, 10) | border;
+    return vbox(std::move(content))
+      | frame
+      | size(HEIGHT, LESS_THAN, 10)
+      | border;
   }
 
  private:

examples/component/gallery.cpp

@@ -54,7 +54,12 @@ class MyComponent : public Component {
   }
 
   Element Render(std::wstring name, Component& component) {
-    return hbox(text(name) | size(8,1), separator(), component.Render());
+    return
+      hbox(
+        text(name) | size(WIDTH, EQUAL, 8),
+        separator(),
+        component.Render()
+      );
   }
 
   Element Render() override {
@@ -69,7 +74,7 @@ class MyComponent : public Component {
         Render(L"radiobox", radiobox),
         separator(),
         Render(L"input", input)
-      ) | border | frame | size(10,10) | border;
+      ) | border;
   }
 };
 

examples/component/radiobox_in_frame.cpp

@@ -21,7 +21,7 @@ class MyComponent : public Component {
   }
 
   Element Render() override {
-    return radiobox.Render() | frame | size(20,10) | border;
+    return radiobox.Render() | frame | size(HEIGHT, LESS_THAN, 10) | border;
   }
 };
 

examples/component/tab_vertical.cpp

@@ -1,5 +1,4 @@
 #include <iostream>
-#include <thread>
 
 #include "ftxui/component/container.hpp"
 #include "ftxui/component/menu.hpp"

examples/dom/package_manager.cpp

@@ -43,7 +43,7 @@ int main(int argc, const char *argv[])
   int nb_done = 0;
 
   auto to_text = [](int number) {
-    return text(to_wstring(number)) | size(3,1);
+    return text(to_wstring(number)) | size(WIDTH, EQUAL, 3);
   };
 
   auto renderTask = [&](const Task& task) {

examples/dom/spinner.cpp

@@ -19,7 +19,8 @@ int main(int argc, const char *argv[])
       entries.push_back(separator());
       entries.push_back(
         hbox(
-          text(to_wstring(i)) | size(5,1),
+          text(to_wstring(i)) | size(WIDTH, EQUAL, 2),
+          separator(),
           spinner(i, index) | bold
         )
       );