Miscellaneous refactoring (#160)

* Reorganize ContainerBase

- Reduce Container overloads using default arguments
- Extract member function pointers to virtual functions
- Separate classes for Vertical, Horizontal and Tab containers

* Collect unpack from NodeDecorator subclasses

* Reduce redundant expansion for aliases

include/ftxui/dom/node.hpp

@@ -36,7 +36,7 @@ class Node {
   virtual void Render(Screen& screen);
 
  protected:
-  std::vector<Element> children_;
+  Elements children_;
   Requirement requirement_;
   Box box_;
 };