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/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;
   }
 };