Extract common struct Dimensions from Terminal (#171)

- Convert Dimension to namespace to allow defining Fit method from dom.
- Use Dimensions extracted from Terminal as replacement struct.
- Convert Terminal to namespace as it only defines static members.
- Remove dom references from screen library (circular dependency).

include/ftxui/dom/elements.hpp

@@ -8,6 +8,7 @@
 #include "ftxui/screen/box.hpp"
 #include "ftxui/screen/color.hpp"
 #include "ftxui/screen/screen.hpp"
+#include "ftxui/screen/terminal.hpp"
 
 namespace ftxui {
 class Node;
@@ -102,6 +103,10 @@ Element nothing(Element element);
 // combinaison with dbox.
 Element clear_under(Element element);
 
+namespace Dimension {
+Dimensions Fit(Element&);
+}  // namespace Dimension
+
 }  // namespace ftxui
 
 // Make container able to take any number of children as input.