Address felixjulianheitmann suggestions. (#288)

- Remove unused examples/dom/flexbox.cpp
- Add canvas(width, height, void(Canvas&)) method

include/ftxui/dom/elements.hpp

@@ -59,7 +59,9 @@ Element paragraphAlignCenter(std::string text);
 Element paragraphAlignJustify(std::string text);
 Element graph(GraphFunction);
 Element emptyElement();
-Element ElementFrom(ConstRef<Canvas>);
+Element canvas(ConstRef<Canvas>);
+Element canvas(int width, int height, std::function<void(Canvas&)>);
+Element canvas(std::function<void(Canvas&)>);
 
 // -- Decorator ---
 Element bold(Element);